Mick, thanks for the comments, and your guess was close. 800mm from centre of pump to outlet from F/E tank. However, even if I lowered the pump I could only gain a further 300mm without having to re-plumb the HW tank / three way valve and feeds to the rads. As we don't have a loft and the roof is shallow pitched, there isn't much I can do about moving the F/E upwards either..
So far though, the system has been really quiet and no signs of air in the rads
So far though, the system has been really quiet and no signs of air in the rads