Thanks to everyone for your help on this...

and to answer some questions, but not all as we couldn't experiment
with all ideas last night as once things had heated up, you have to
wait for them to cool down again before trying something else!

couldn't find any valves (unless they are hidden under the
floorboards) but certainly nothing near to either boiler, pump, or hot
water tank.

Certainly agree that the boiler could be 20-30 years old.. looks like
it could be - but when we moved in the plumber who's fitting our new
bathroom suite in a couple of weeks looked at the boiler and said -
'yeah, they're tops.. nothing to go wrong on them - so don't replace
it unless you have too' which implied it was old, but ok.

pump appears to drag water around the system (cold side of boiler) and
you can definitley feel it start when everything is cool and you turn
the thermostat up.

Just the hot water on, does heat most rads.. overriding the thermostat
for central heating.

anyway, we're going to replace the very old timer (to give independant
c/h and water heating) and have a digital thermostat...

only have the water heating for a couple of hours a day, as opposed to
most of the time we have the c/h on ( baby in the house so need to
keep it ticking over)

and hopefully the time that the water being heated 'overrides' the
central heating heat will be greatly reduced..

else it's going to be a rip out and replace i suppose...