I had a 205 git that refused to give up it's filter even after shoving a screwdriver through it.
I've tried strap wrenches, chain wrenches and alternatives (old cam belt + spanner + ingenuity can be ok)
What I find the only sure method though for when all else fails is a filter wrench - basically a pair of mole (vice) grips with a wide jaw. You clamp it round teh filter near the engine end, and then just turn it like you would a spanner. Space to use one can be a
problem with some configurations, but I know there's enough space in a GTi
Cupra Sport's bonnet. Just used one on Friday... Shouldn't be much more than five or six quid in a motor factors'.
As already posted, if the filter's been put on properly and not then left there too long it really ought to come off by hand alone though, and this should always be your first approach. Any tool risks damaging a filter, at which point you have no option but to finish the job properly before you can start your car again!