John wrote:
If you have replaced the float valve AND there is no sign of it dripping
when up to level then you may have a leak in the coil inside the cylinder
which is allowing water from the heating system to make its way through into
the hot water (tap) side. This may be evident from a dribble from the
heating header tanks ball float valve without attendant rising level in that
tank.
If this is the case then you will have to replace the cylinder. Continued
leakage will cause damage to the inside of your boiler and rads etc and
contaminate your hot tap water

John - thanks. Tonight I'll stop the cold water feed to the feeder tank
just to double double confirm that it's not coming from the float valve
(I'm 99% sure it's not, but could be that the replacement, though new,
is faulty)

Andrew.