Thanks Lynn & Brian. Regarding the manual, I did find that one, but wasn't sure if the parts would be the same as my 206-RB-2. In fact, I don't think all the parts are the same as I just ordered needle bar screws from that manual and they are the wrong fit.
Regarding the problem I had, it is now resolved - although I think I need to tinker with the tension as Lynn said as I do stitch some very heavy sections and can never get it right.
The problem was in the timing. I adjusted the needle bar until the hook stroke was lined up, but then found that the needle was hitting the bobbin case, so I rotated the hook assembly instead and it was way out. I've run a load of test pieces and no missed stitches, so perfect
Thanks for your help,
Rob