Mountfield RV40 Lawnmower - No spark
 

I've a problem with a Mountfield mower, not done much work, about 2yrs old. Sometimes starts first time, runs for about 1-2 mins, then cuts out, then nothing, wont start, plenty of fuel coming through, but no spark?
Any thoughts?

Just noticed the magnet & flywheel touch at certain points as I turn it, does it have to have a gap all the way round as it's spun?

Did some more investigation today.
When cold it starts fine, and cuts ok, if stopped and started within 1-2 mins, it starts ok, but if left for any more time (2-5 mins) it won't start until it cools after about 3/4hr-1hr!
Any thoughts what it could be?