Sorry for delay - line fault...
Very little is condensation - much is due to water leaving downpipes and having nowhere to go! No drains... We're hoping to build an extension one day, so will have to dig up concrete and install drains then - till then, we'll collect the water in butts and use it... though they might overflow in heavy rain, at least we'll be no worse off than now.
The rest is presumably rising damp - half the world (as asked by us, that is) advises a standard injected damp-proof course (and Thoraseal-type plaster replacement), the rest seem to advise leaving it. The previous owners hacked off the plaster every few years and replaced it, along with the skirting boards. The house doesn't smell damp (or hasn't in the four weeks we've been here!) - but the skirting boards are the most rotten pieces of timbers I've ever seen...! And papers left on the floor in places go moist. I don't want the furniture to go mouldy!

That sounds like excellent advice, which I shall follow... Thanks.

Do they ever work? Is there ever damp that they will stop? Usually the dpc company specifies strong waterproof cement render inside up the walls . This itself stops damp but not condensation. Once walls are wet they need to be able to dry out first before plastering with cement possibly 18 month job that!

Best plan may have been to lime plaster it -- then no problems with damp (except may be badly leaking rainwater goods or plumbing). It will always be dry too.