How many of the large blank outer walls face southeast, south or southwest?

At first sight, not much good for solar then. Panels need to face within that southern quadrant, so they'd have to stick out from the wall on brackets, so they can be turned to face southish. They should also tilt upward, between 25 and 65 degrees from vertical. Unless you had very large low cost panels flat against your northeast wall, that would only be expected work at low efficacy on diffuse, not direct radiation. That's what panels do on an overcast UK day anyway, regardless of ideal orientation. Why not put the panels on the roof?

I'd say wait till photovoltaic panels get cheap/fuels get expensive. PV will do everything that a wet panel will do, and much more. Not everone will agree with this!

S'funny, PVs attract such empassioned scepticism, when it's suggested they may get cheaper by quantum leaps and/or that other fuel prices may rocket to make PV electricity look cheap. My perception is that all hi-tec technologies start prohibitively expensive, eventually entering exponential price-fall if production ramps up. Not so pronounced for mech-eng products like wet panels, but very much so for electronics. And PVs are essentially electronic components, especially the latest chassis-less ones. And especially when potentially limitless demand creates ever more sophisticated, low-cost, high-volume manufacturing and massive R&D results in improving performance from exponentially-diminishing quantities of the hi-tec, possibly rare active materials. And there's other ways energy-density is improving e.g. http://www.wired.com/wired/archive/13.07/solar.html. So it's bound to come and will make wet panels and all their plumbing and mechanical kit look really antiquated. I can't tell when, though - what's best guess?

The matching technology, that's still rudimentary, is no-moving-parts electricity generation from temperature differentials by thermocouple. Ideal, maybe one day, for recovering waste heat from air- and water-flows, but almost-invisible efficacy so far.

One of the biggest advantages of electricity-borne energy is that its output can be at any temperature you like. Whereas the output temperature of air- or water-borne energy-transmission systems (or any other relying on the movement of heated material) is limited to something less than the input temperature, unless 'pumped up' by a heat pump.

Thus on a not-so-sunny day, flow temp from a wet panel is only medium and can only heat the storage tank up to a less-than-medium temperature, however long you wait, regardless of any surplus collection potential that the tank becomes incapable of absorbing. Whereas an immersion heater powered from a PV source can be configured to run any desired temperature, e.g. well above 100oC, so the stored water can eventually reach say 95oC, as fast as the PV can collect energy, of which it readily absorb all.

Nigel, what you say about collection efficiency and the resultant energy presently available, I believe is right - but that doesn't contradict what I said. I'm talking about the temperature at which whatever energy is collected can be outputted, regardless of whether that energy is a lot or a little after collection efficiency has had its effect,.

Posted By: nigelAnd why is the output temperature relevant?

Posted By: fostertomE.g. on a not-so-sunny day, glycol flow temp from a wet panel is only medium (say 60oC) and can only heat the storage tank up to a less-than-medium temperature (say 55oC), however long you wait, regardless of any surplus collection potential that the tank becomes incapable of absorbing (incapable because delta-t between the stored water temp and the glycol temp drops to only a few degrees (60-55=5oC), as stored water temp creeps up to almost-glycol temp - rate of heat transfer is proportional to delta-t so diminishes to almost nothing). Whereas an immersion heater powered from a PV source can be configured to run any desired temperature (say 150oC), so the stored water can eventually reach say 95oC, as fast as the PV can collect energy, of which it readily absorb all (readily because delta-t between the stored water and the immersion heater remains high (150-95=55oC), even when the stored water approaches its max temp).

PS I've edited my quote for clarity.

Posted By: nigelwhy do you need water above 60oC ?....ever heard of a pv hot water system?

Agreed wet panels are currently ideal for domestic hw, inexpensive, mature, good payback. Collection efficiency is OK because, not requiring very hot stored water, delta t is adequate, creating adequate rate of heat transfer. For just that, for now, look no further (and swimming pools - even higher efficiency because larger delta t)
For contribution to space heating, wet panels are marginal, lots of machinery, expensive tanks, which should really be special stratifying ones to get better/hotter storage. Ideally, the most sophisticated/expensive panels to get hotter glycol flow temp. Panels' efficiency decreases ever more sharply, to PV levels or worse, as you try to get hotter stored water, because delta t decreases. With PV, no such problems, efficiency (such as it is) is maintained, great simplicity, easy accurate control; except current prohibitive price of the panels themselves.
For really hot, therefore dense storage of heat for longer periods, forget wet panels; PVs are ideal for it, same comments as previous. For my pet idea of interseasonal storage deep underground, despite PaulinMontreal's constructive criticism of the idea, again PVs seem to me to leave any wet system standing.

Posted By: nigeldont tell me that there is suddenly going to be a seachange in pv efficiency

Efficiency's creeping up all the time, 2 generations of new technologies in the pipeline, foolish to say there won't be more, plus things like http://www.wired.com/wired/archive/13.07/solar.html increase overall system efficience, or at least energy density. I also said why cost will fall exponentially, 6 posts back. And that other fuel prices will rise sharply, making PV look cheap (along with other renewables).

Above all, PV looks to me like the coming key to inter-seasonal storage, even if collection efficiency still by then neccesitates large areas of PV, providing it gets cheap enough. Daft to ignore that possibility.

Has anyone ever heard of a pv hot water system?

Yes Solar Century have developed one its called C21 I think.

The water cools the PV side making it run at its most significant

Julia, most photovoltaic and solar thermal systems are available for mounting either on a pitched roof or on racks on a flat roof - what does your building have? Photo voltaic panels generate electricity directly. As Nigel and Tom point out they are not very efficient and they are expensive. But they will generate electricity whenever there is daylight. Any surplus energy you don't use can be be 'sold' back into the national grid. Solar thermal systems are generally either available as flat panel (imagine a radiator painted black) or vacuum tube (like the vacuum unit in a thermos so not much affected by the surrounding air temperature). Generally anti freeze is pumped through the units where it becomes heated and that heat is then transferred into your hot water cylinder. In both cases it is usually going to involve a new hot water cylinder plus the controls and plumbing to connect to existing. Flat panel collectors are generally less effecient than vacuum tube systems but they are also quite a bit cheaper. Tube systems are quicker to start working and will generally produce more hot water through the winter when the sun is lower because of their shape and the effect of the vacuum.

Low Carbon Buildings Programme can give grants to help with the cost of PV but at the moment I think those for solar thermal have been temporarily suspended. There are a small number of British companies actually making solar panels in the UK - www.aesolar.co.uk , www.filsol.co.uk . The better tube systems seem to be European made - Schott and Wiessmann being two examples - where the tube is made from a single piece of glass so there is no seal to fail. My feeling is to avoid the cheaper end of the (imported) tube market as these products may be prone to fail more easily.

Good luck with your project - is your building ACME by any chance? I used to have an ACME studio in Arrow Rd.
Julian