Problem solved and understood now. Take a look at my picture and you'll see that where the mushroom 'peg' goes into the bumper the shoulder is about 5 mm deep after it widens. On Paul's it widens but no shoulder. So... cut 5 mm off and its fine. We also filed the shoulder off which I now realised wasn't needed, and drilled a hole to take the metal centre pin rather than sawing it all off. See below if that sound confusing, but basically, there are two types of mushroom peg lads!!!





and job done