Being a mechanically minded bod (I still wake in the night with cold sweats remembering my electrical systems lectures...), I was wondering whether anyone had any experience of this system or any of a similar ilk?

http://www.theengineer.co.uk/Articles/306208/VPhase+energy+saver.htm

It seems like quite a bold claim, and they usually turn out to be red herrings but is there any truth in it?

Is there any truth in it? Yes and no.

Yes it should work, the question is how well. Some devices do burn less power at lower voltages but many don't. Take a kettle for example. The amount of energy it uses depends on the amount of water in it. If you lower the voltage the power consumed may go down but it will take longer to boil so you would expect no net saving.

Here..
http://www.buildingtalk.com/news/vph/vph102.html
they claim ..

Using the low cost VPhase VX1 to manage the voltage to a pre-determined level towards the lower parameter means that the energy consumption of many household appliances can be reduced by anything between 5% and 18%".

Devices that exhibit energy savings when voltages are reduced include refrigeration appliances, washing machines, dish washers, tumble dryers, vacuum cleaners, other devices using motors, air-conditioning units, central heating pumps and lighting (including energy saving lightbulbs).

Heating devices such as ovens and electric showers wouldn't show any savings but these high current appliances are usually connected through other stand alone circuits.

That last bit is possibly a bit optimistic... To keep the cost down they will want to fit the vphase between the meter and consumer unit where there are just three wires to deal with. If they have to start opening up consumer units and rewiring them so that the oven is on the meter the meter side of the vphase device then the installation will be a lot more expensive.

I'm surprised the include dishwashers on the list. I would have thought heating the water and drying the dishes was the biggest use of power not turning the sprinkler head.

Before you fit one you should survey your supply voltage. It's possible you live in an area where the voltage is already at the lower limit so there might be no or only a minimal saving to be had. On the other hand you may live in an area where it's high or low at certain times of the day. I'd want monitor it over a week or so.

These things are normally called AVRs (Automatic Voltage Regulators) which were used extensively in broadcast transmitters, however were rated at 250kW or so. These diddy little things will help filament bulbs have an increased life (or illumination) but themselves are a source of unreliability. If it fails, do the lights go out? Unless you are conducting some sort of experiment that requires a stabilized mains supply , forget them!
frank

If you live on the end of a long feed (1 mile) of insubstantial cable shared with six other households, like I do the mains is typically 245V at mid night, going down to 205V when I turn my electric shower on.
My point is that the mains is rubbish in some parts of this country and we survive. Most electronic devices that require a regulated voltage have an internal regulator, which can normaly cope with a wide range of voltages.
There is a situation called "brown out" when the 230V falls to something like 170V. These sort of gadgets can't cope with this wide variaton, but to be fair they would increase the 170v to 204V. The generators/power distribution kit can't deal with this voltage either so something "pops" and you lose the lot, or the generator is taken off line (no supply again) or some one hits the generators regulator with a 14lb lump hammer and the voltage returns to its nominal. Its better to fix the faults at source or go for a UPS (Uninteruptible Power Supply) which will supply a proper local supply, until the local mains has disappeared for some significant time (10min-> 2 Hrs?)
Frank