Which one, there are at least 4
The ones on the back of the block will incress your vocab, but are doable, make sure the block hasn't rotted away so the new one can seal, if has has rotted, go to homebase and buy some central heating sealant, can't remember what it's called, but it comes in a yellow and blue tube/box.
The other plug sits inside the bell housing, I think, or just above, and is a box out job.
Oh, and it's not a GTi, unless it has fuel injection.
kraM