Level in F&E tank went from near empty to near full overnight with boiler switched off, so I think I have to conclude it's the coil.
Do all HW tanks (mine seems to be 450 x 900) have the inlets and outlets all in the same place, or are they all different just to make my life more difficult?
As to whether I will have other problems, I guess until I fit the new tank, I won't know....