Hi, thank you for the reply.
Curiously that was my first thought.
However, I replaced the gear shift for a Touratech lever a year or so ago (as the old one was loose and worn) and the new one sits extremely tightly to the shaft. I've checked it when it's doing its 'I won't go into first gear' thing and it's not striking the frame at all.
Also, if this was the case, wouldn't it do it all the time?