What are the rules to take into account when siting a header tank for a
conventional pumped CH system. I haven't been able to find any hard and
fast guidelines .

I have had problems with heated towel radiators and tall bathroom rads
that don't get hot at the top and wondering if this is due to height of
header tank.

ALl other rads work fine.

The distance from the water level in the tank to the top of the tallest
radiator is approx 1 metre.

Would this be sufficient ? I have 13 rads and 2 heated towel rails
(heated off hot water circuit)

ANy advice welcome.