How bad is bad? Is it lots of light muck or thicker oily stuff thats been there for yonks?
Gunk type degreasers do work but they stink for ages afterwards and unless it's horrendously thick I'd just go with G101 and a brush. To aid it you can carefully use a jet wash at the garage on a hot setting (unless you have a hot PW at home) but be very conservative with where you squirt it avoiding electric plugs, air filters etc or stick a bag over them. The heat certainly helps (my sump was pretty bad but came up a with the jet wash) and removes it rather than spread it round (your drive included!) but just watch what your'e doing.
**edit** just to add, I've pressure washed my Mk3 a couple of times and would say it seems prone to getting water where it shouldn't. Keep the engine running as you do it and it'll be drying itself as it goes and alert you as soon as you get carried away. Otherwise you run the risk of loooking like a wally when it won't start < previous :embarassed: