Ian & Hilda Dedic wrote:

it is a new tap, that seems to be the problem....

it's a franke pull out spray jobby....

we only really need a bit of a boost in hot pressure to this one tap,
the other mixers in the house seem fine so I wonder if looking at the
inner workings might be a possible way to go.

tap experts anyone?

I had the same problem, after buying an Ikea pull out spray type, which is
probably the most unsuitable tap possible for a low pressure hot water system!
I bought the Grundfos home booster pump, and while not perfect, it was a
better solution than buying a new tap as I had already fitted it to the sink!

Without the pump fitted, although the hot would run (very slowly), any amount
of mixing with the cold forced cold water backup the hot pipe and back into
the hot water cylinder. This is because of the pullout spray nozzle meaning
the mixing is done within the tap body rather than having separate hot and
cold channels exiting at the end of the tap.
So, in addition to the pump I fitted a single check valve to the hot pipe
under the sink. This was not without problems though, as this restricted the
flow too much so the pump did not trigger automatically! I removed the return
spring from the valve, and all is well.

The pump is totally silent in use, the only problem is that when you want to
run the hot, or mix with cold, it works best if you switch on the hot to full
until the pump comes on, then close down the flow and/or mix as necessary.
Luckily when I fitted the kitchen I installed mains sockets in the back of
every cupboard just for this type of eventuality, and so I installed the pump
in the cupboard under the sink.

Cheers,
Ben