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I overlooked a small point Martin. You say that payback time can be as little as 5 years!!!

This equates to a 'hot water cost saving of £600!!! per year'. Some mistake here surely? With respect, I would'nt advertise that, bit risky.

I have looked at my own electricity bill, for 'all' the electricity used in my household over a year. A large hard to heat house, some use of a fan heater, electric water heating, electric cooker, and all the other bits and pieces including 2 teenage daughters who leave lights on. I can not equate in any way how you could ever get to '£600 savings on hot water costs alone'. This is a first in the industry I would think.

John

easy! - if all you have to heat your water is an immersion heater in an unlagged tank (and many people still do!), it'll use a 2kw immersion heater at 20p per hour - say 15 hours a day - three quid per day - 200 days or so - bingo, 600 quid!
So, it is eminently possible to pay back in 5 years in that circumstance!
I did also say that in most cases it'd probably be nearer ten years! - none of us have crystal balls, I haven't a clue what will happen to fuel prices, I think they'll just go on upwards, so any projected payback time can only be conjecture
One of the biggest factors in payback is how much it's cost you - the "industry standard" round these parts is anything from 6-11k for a simple system (often disastrously simple!) - If you do the sums on those figures, payback is gettiing silly!
Every circumstance is different - we deal with a lot of country properties that often have a rudimentary heating system based on a rayburn or ancient oil-fired boiler, and they often use an immersion heater, or bottled gas to heat their water in summer - it does pay back well there!
As to Legionella, you'll find that it is recommended to keep all stored water at above 60 degrees to "nuke" things like that - you don't have to boil it!
And as for adding value, according to three local estate agents, it already does, and will do so more and more as there are moves afoot to make these things declareable when you sell! If the industry standard price is high, that is the price it will increase the value of the property by - if you've got it done for half or less, you're laughing!
As for all that tosh about them being expensive in ongoing costs is just that! As mentioned earlier in the post, a pump will probably use 3 quid's worth of electricity over a year - a new pump will cost around 35 quid if it goes)mains pumps are very reliable, cheap and easy to manufacture, and a litre of antifreeze every five years is not going to break the bank either!