I investigated a bit more, I pushed the ball through slowly with my finger and saw it was getting stuck between the 2 metal guides, I could just let it rest there. This is before the ball even gets to the new piece so wasn't an issue with that, although I noticed you can also get it stuck on the new diverter piece at very low speeds which probably doesn't help. I adjusted the outer guide so the ball doesn't get stuck anymore but unfortunately it's made no difference at all, thought I'd found the problem. I may try a stronger coil but will see what stern say first, if they're even responding to UXM messages anymore, probably sick of them
Let me know if you manage to fix it please Kirk