Hi Gavin
Totally new to this game as well but am having the same issue with 2 inch EP on my new SW 14 dob. My 40mm wild field piece would not focus at all. Got myself a extension sleeve but found it wouldnt fit in the focuser, it was too wide by a good 2mm
. After some experimentation with another two found one that fits the bill. So make sure the shop knows exactly what size one you need as it was really frustrating having to go back and forth till i got the right one. Cost me a week and a half of observing time. Was worth it in the end though fantastic super bright wide field veiwing at approx 30x. Hope his helps
Cheers Matt