Funnily enough i had this exact problem sorted on mine last week. I went to work and on the way i couldn't go past 2nd gear, at first it felt like i was in top gear with nowhere else to go, so a bit of clonking and it then jumped up to 3rd. Then a bit further on it did it again, and i noticed that it had slightly gone past 2nd into a non existent place, almost like it had found a little place sideways, if that makes sense? Gave it a slight tap down and it was properly into 2nd again then it was hit or miss with a bit more clonking to go back up to 3rd.
I was told that the 'selector drum detent wheel' was slightly worn, and should really be replaced, and while it was apart to change a spring as well. Whether it's the one you're saying i'm not actually sure, as i let the mechanic get on with it?
What actually happened was originally a 1 week wait from Fowlers for the 'detent wheel', which then turned into a 4 week wait after a phone call from them.
There was no way i could wait that long so the bike was put back together with just the spring changed and it HAS sorted it out.
In fact i was told afterwards to get a new clutch cable as well, which i did, and now the bike changes gear so smoothly it feels like a brand new bike!!
