Right, so my charge fuse blew very gently and melted all over the fuse box. The ignition fuse has some melted plastic. I can replace the fuses and run the bike, but the charge fuse is noticably hot if I whip it out straight after.

So, I presumably have a short somewhere. Trouble is I don't know where to start. Any suggestions for places to look / wiring to replace? I've bought a replacement fuse box.

Have you had any thing else give any trouble recenly or fitted anything to the bike ???

It could be some damaged wiring or something like an over powered headlight bulb but I reckon it's more likely a dirty / corroded block connector.
It's a boring job but somehow quite teraputic, open each block connector, clean the contacts, put a very small amount of vaseline on them and do the connector back up. If you want to find the one more responsable for the fuse have a look at the reg rec connector.

Awesome answer, thanks Evil.

I haven't added anything to the bike, but it's only been up and running after being sat for years, which makes dirty connectors all the more plausible.

I'm having to run it in as well, so I think it's because I've upped the max revs from 7 to 8 that's done it.

Got it I think! Replaced the fuse box and chopped some seriously shitty wires, and gave the reg rec connectors a good clean and some vaseline.

Thanks very much! point to you, Mr Chicken

Glad it worked