What revision is your car?
It makes a big difference to the work involved, as the rev1 & 2 has to have the exhaust downpipe removed (I think, I could be wrong though).
it's easy.
remove oil.
remove center section of exhaust.
undo the 12 or so bolts that hold the sump on, pull the sump off without damaging the block.
If it's a turbo then you will have to remove the oil feed pipe too.
Get some instant gasket from Halfords and replace new sump!
Cheers,
Paul.
ive got a sump, engine has a hole in the side you can get ur fist into., engine still in the car, if you want it, just come and get it. else its going to the breakers in few weeks