I used something called Tiger Seal, really good stuff, can get it from Halfords for around £12. Didn't need to remove the bumper, you just need to jack the rear end up and undo 2 bolts from either side, 2 bolts in the middle, and the it will un-clip from the bumper itself.
You'll have to use glue unless the new diffuser comes with clips like the original, in which case you can just clip it in. The Maxton one doesn't, so it has to be glued in.