fuel tank

The tank is not hard to take down but I suggest that the tank be professionally repaired. There are companies out there that will repair and reseal the tank in side and out.

The leak is likely to be through rust developed by the heat shield sitting above the tank to protect from exhaust heat.

After repairs, my suggestion would be to wrap the exhaust with heat proof bandage rather than matting sit on the tank. Or get heat shielding that will not retain moisture when stuck on. The Mercedes Benz patch is made  of fiber glass matting which does retain moisture. You can not get new metal tanks, instead there is a plastic replacement which is expensive as is its fitting kit.

Divert the exhaust  before it gets to the tank, exit at the side like a w463. If the plastic has melted you are fortunate not to have gone up in flames

Unless the hole is huge then it is 100% repairable.

You need to take it to a company that does ultrasonic  welding (not one to try at home with a DIY soldering gun based kit!).

I used these guys http://www.iep-ltd.co.uk/ to repair a puncture hole (posi screwdriver) in the bottom of mine.

They cut out a circular section, welded it and you wouldn't be able to tell it was done, it's that good. The advantage of ABS of course is that that the weld is at molecular level bonded in as part of the plastic, so once done it structurally as good as new too.

Re. Exhaust exit - something to consider is that side exit is more prone to damage if offroad.  

You can buy a new heatshield from MB for about £100 - I've also wrapped my exhaust across that section which makes the temperature very much lower (almost put your hand on it). Lots of people sell wrap kits, expect to pay about £30 for one.

Example:

http://www.thermalvelocity.co.uk/Exhaust-Wrap-/c-1-105/?gclid=CN2kkq2glb...