It is quite rare for TTRs to leak from the neutral switch. Can you tell if it is leaking (a) from the switch's washer where it goes into the crankcase or (b) from the centre where the electrode comes out?
My guess is that it's the latter which means you need to source another switch
Thanks Brian - it looks like its the gasket or seal behind, so I've got one on order now. Hopefully its not the switch but at least it is a fairly available part looks like.