So you get the most hot water for heating on the sunniest days when
you are least likely to use it. I think you have it arse about face.
Use solar to augment the hot water which you will use year round.

MBQ

I see where you are coming from. Isn't that the conundrum with any
domestic solar-based heat / energy colleciton in the UK? The
conditions that will lead to "usable" heat from the panels will tend
to be those days when you don't need the space heating. That's why in
my original post I asked about heating water by solar and
supplementing it with gas.However, one always needs hot water, even in
summer for personal washing (unless that isn't your thing) and clothes
washing.

Whether it's financially viable, I don't know. At least I will have
the smug satisfaction of feeling I am "doing my bit" for the
environment. I am sure there is an argument that the energy expended
in manufacturing, transporting and installing the equipment needed for
a "solar-supplemented" system far exceed any marginal savings in gas
burned.

thanks for the link about the "thermal store". That's looks like the
kind of system I might get. Looking at the diagrams and descriptions
it seems broadly similar to what I described in my original post: dual
"heating" circuit from solar and the boiler to "top-up". Substitutue
"tank/cylinder" for thermal store and it seems a similar idea. Anyway,
I now have the right terminology and a potential supplier - so much
thanks for that.

Thanks for the good point challenging the idea of replacing the old
boiler with a combi. With the thermal store, system as I understand
it, the boiler is simply a source of heat to top up the thermal
store.

Oh, I'll ignore the high-handed tone of much of the responses in this
thread. I had a few ideas, I came to this group asking for information
and advice in the knowledge others were better informed, otherwise why
would I come here? The information provided has been very useful to
me. Thank-you. I'll ignore the disparaging tone in which much of the
information was delivered.