Changed the cambelt myself recently without any previous cambelt experience, so it's not impossible! I would however be fairly familiar with the engine at this stage having replaced sparks and other jobs previously.
The aux pulley nut is an issue. The pulley has to come off as you can't access the cambelt otherwise. I used a 3/4 drive breaker bar and the nut came off easily enough to my pleasant surprise. But I then needed to buy a pulling tool to get the actual pulley off as it was jammed tight.
Its difficult to get at the water pump on the 156 (you might have more space on the 166 though). To remove the water pump pulley I had to loosen the front engine mount and lower the engine slightly using a jack for support.
As long as you have the right tools (allen keys, lots of spanners, cam locks, tensioning tool, cam pulley holder etc) and are methodical, and have some experience with spanners it is doable. But it took me a very long number of hours to finish (I'd estimate about 24 in total.. over 4 weekends!).
hope that helps..
Last edited by GavinH; 02-01-10 at 11:51.