[Nickj]

You could leave it dry in the sun to dry out for a few days, then steam clean the inside, then fill it with a strong detergent soloution for a few days, then drain and steam wash.
You still might not remove the residues in thicker 'varnish' deposits.
All you need is the equivalent of a teaspoon or less of fuel or anything vapourised to make an impressive bang, it will also reshape the tank a little.

That really leaves you with a few options which are:
Sod it, clean as well as possible and hope it is clean enough (or get a specialist to do it)
++ Expensive and potentially dangerous, potential for local distortion of the tank during welding. Will need a respray afterwards so the rest of the bike looks even worse so you end up having to paint everything,

Clean it as well as possible and tin solder in a patch, no a teeny Halfords soldering iron isn't going to cut it here. You need big copper or steel bits and a big heat source to get them up to temperature.
+++ Very old school stuff and a right PIA to do using very big very hot lumps of metal to provide the heat and a mini forge, big propane burner or oxy-acetylene to heat your bits (Oh and very thick welders gloves). Still need to do a bit of filling post fix and respray so again that will lead to more re-painting.

Clean it as well as you can, rattle a few rocks around the inside then treat the porous area with some rust remover stabiliser and then (as per Chriswilly) seal the inside of the tank with any of the fuel proof epoxy sealers. You'll get a 1 - 2 mm layer of epoxy over the leaky section and stop it recurring anywhere else. Any of the now treated rust acts as a reinforcement for the expoxy so it will be much stronger and stable.
+++ Easy to do, very limited skills required and a pretty permanent fix. Probably just need a local touch up on the tank when you've done.