Bodger67,
From one of my earlier posts re my pumping over problem(a search would have found it for you);
Had the same problem when the pump was on any higher setting than 1, even then I got steam coming out of the expansion pipe into the tank, condensing in the cooler overflow pipe causing it to constantly drip. Have now fixed it and can run pump upto speed 3 without overpumping, 2 fixes but both addressing the same problem;
1) Water level in expansion tank was too high, re-adjusted the float to put water level just above the low pipe feed at the bottom of the tank (because this is an open system, the level of water in the expansion pipe will be the same as that in the tank, therefore, if it is high, it will take very little expansion of the water in the pipe for it to flow into the tank.
2) Expansion pipe into tank was too low, only just about 6 inches above tank before bending through 180 degrees into tank. Same as point 1 above, very little expansion still required before water flowed into tank. Increased the height of this 'bend' so that it is approx 3' above tank before the 180 turn back down to tank.
Sorted it for me, hope it is of some help....