Same with me, it's a bit fiddly getting the TDC off but just clean the magnetic tip until it's shiny, mine has been OK for 3 years now with just the clean.
Sorry for the delay in letting you all know how it went.
I didn't want to blow any trumpet till I was sure I had a result.
Took it off last Thursday evening and gave it a good clean. I didn't realise my hands were that big till I tried to get them into that wee space. There were only a few particles of metal around the tip but removed them anyway, and put it all back together.
To my surprise started first time, and even better is that it is now idling better (around 750rpm).
It may still all go wrong again (in fact it probably will just after I click submit), but thanks to all who responded, and also those that contributed to the how to.