So got hold of some pushfit fittings from Screwfix yesterday and they are too big by 1mm ?
Geberit OD 39.8mm
Floplast pushfit pipe OD 40.9mm
the 1mm I thought would not make much of a difference but the Geberit exit pipe flaps around like a d**k up a shirt sleeve.
My own stupid fault for not taking the Geberit with me, ah well.
Does anyone have any other ideas please because I'm about to send this thing into the back field