I've done a trawl of the archive and I've found some answers, but in
some ways some more questions.

I've a 12 metre plus run to the kitchen sink from the HW tank with
only a 3 - 4 metre head, and have compounded the problem by fitting a
pull out mixer tap which has a high insertion loss. Various solutions
have been played with but a local heat source off the mains seems the
best option.

Just how good are under-sink heaters ? what is the storage types ?
why do they vary in power from 2.2 to 9.5Kw? What is the problem with
expansion - I would have just a short pipe run to the mixer tap ?
Would I be best to arrange a longer run ? I've seen something about
vented taps - how do these things fit in with an ordinary mixer?

Any help would be appreciated guys, many thanks.

Rob