Cheers for the advice guys, however I gave up and got the drill out
Took 10 mins when I started with more violent tactics, will post a pic of what I did incase anyone else has same prob.
Basically I took a Colbolt drill bit and drilled 5 holes directly above the allen-key bolts holding the cap on the tank (only need to be around 5mm), poped in the allen-key, loosened and hay-presto, off it popped
Just have to be careful not to skate across the tank, use new drill bits and no cheap rubbish!