If you read the sales schpiel with a magnifying glass, you will find in a back corner somewhere, that the 20% additional savings are measured over the savings compared to a se boiler i.e. se=80% efficiency, he=90% efficiency.
So it is not ( up to ) 20% extra off the gasbill, but 20% of 10% = 2% off the gasbill.
That means it is extra money to buy, extra money to install, and an extra unit that can go wrong.
Great for me as an installer, but is it worth the hassle for you to get up to 2% reduction on your gasbill?
Personally, I don't promote it.
You want to save money on your gasbill?
1. Draft proofing.
3. Turn the roomstat down 1 degree, and save 4%
4. Turn your trv's down half a point once a week, until you find certain rooms are a bit too cold. Turn those up half a notch again.
80% of trv's are constantly on max and save nothing