So today its just hot water being timed on - and hence am getting
overflow, upthrough the expansion tank and hence into overflow and then
coming out the pipe at the side of my house.

Can you just be clear that this is the feed and expansion tank for the
boiler circuit and not the cold water cistern for the hot water cylinder?

If so, and there is no water coming through the valve, then I agree with the
other poster that your hot water cylinder heat exchanger is holed. However,
it could also be someone limited in understanding setting the standard water
level too high (it should be very low in the tank), so it is unable to
accept expansion. When cold the water level should be very near the bottom
of the tank. When the system heats up, the water level rises, but not up to
the overflow.

In any case, you might want to remove the F&E tank and replace with a sealed
pressurised system. You'll still need to replace the hot water tank, though,
if it is bust. Try asking about what type to replace it with if you want to
see everyone groan.

If you only have one tank and it is the main cold water feed, and you don't
have a Primatic abomination, then you probably already have a sealed
pressurised system. In which case, the overflow is either caused by:

(a) the water level being too high, so is unable to accept the expansion of
water in the hot water cylinder when it heats up. The water level should be
fairly high in the main tank, but still with expansion room.

(b) again a leak in the heat exchanger. This would be coupled with a
tendency to rapidly lose pressure on the primary circuit, requiring frequent
topping up.

Christian.