Can't unlock water cap

[flutepiece]

I have just bought a 1985 VW Karisma. I have four keys. Ignition key does all the doors. petrol cap has a key. Two others do nothing so far. I cannot get any of these keys into the side water cap lock. Any ideas on how to proceed? What would you expect the other keys to do?

[skippymoss]

Should be able to unscrew the water filler from outside, disconnect it from the hose that feeds the water tank and replace it.
As usual Brickies do 'em.

[happybus]

Has the same problem. Once I identified the correct key(I had it all along) I realised that the lock was not for turning. As it turned out the barrel was completely rusted. I removed the cap with a good twist from a vice grip. Obviously broke the lock. So had to pita new cap on. Not an easy find, in the end had to bodge something together.