Yes the Maxi fuse as they are known if its like a giant Blade fuse, its only job is to protect from a regulator/Rectifier failure so the only thing passing through it is the charging current on its way to the battery
Update: Despite being charged up on Saturday, bike wouldn't start on Sunday morning after the above posting.
Had a serious think and realised that since my trickle charger had stopped working a couple of months ago, I had been using a car charger to charge the battery.....yes, I know...how dumb is that! Nipped to bike shop and bought a new battery and a new trickle charger. While I was out I also bought a Halfords earth lead to slave between -ve terminal and engine (leaving original in place) and new maxi fuse.
All went well at Brands on Monday - bike started no probs and lasted all day just fine. Clocked 13.9V whilst exiting Clearways...this is my new measure of track success!!
Moral: Don't eff about with car chargers, they fry your battery. Cost me 95 quid to learn this obvious fact!!