Dont do it mate! I once went to get it done, booked it in and everything - and the guy point blank refused to touch it as soon as the bonnet was open!! He said it would ruin the car!
So I took myself of to Halfords, got an "engine cleaning kit" and lots of Tesco Value toothbrushes - and hey presto! I wouldnt spray a hosepipe in there either, like the instructions say. Just spray the cleaner (next best thing to paintstripper, so keep away from body work/painted areas) on the toothbrush, give it a scrub. Then wet the toothbrush in clean water and scrub again to get the stuff off. Finally use an old hand-towel to soak up the excess water. Hey presto - clean engine.
P.S. I t helped that my car had had an engine re-build before I did it!!