Nic,
Not a big job, just a bit fiddly to get the lock mechanism out. You need to toake the door card off, and release the lock from the door frame - unfourtunatly this involves moving almost everything else inside the door!
Once you have the lock out, it is pretty easy to fit the new spring - I still have the instructions from when I did mine, I can scan them in if you like (you will have to wait till I'm at work tomorrow though!)
As I say it's not complicated, just a bit fiddly, and you will be surprised how much of the door needs to come apart!
Cheers
Mark