I was having the same problem - an engineer told me the trick.
1) get the got the belt over the drum. You do this by undoing the back panel and pulling the drum back (still attached to the back panel) and slip the new belt over from the front of the drum.
2) belt goes groove side down on drum.
3) the motor has a grooved spindle put the belt over this.
4) at this point the belt is almost right but a bit slack and I thought it was the wrong length or some people on this site have been fiddling around moving the spring on the motor but....
5) the trick now is that the motor is sprung giving resistance as you push it to the right. Push the motor to the right. Now pull the belt to the other side of the black pulley so as the motor springs back it put the belt in tension.
The belt now follows a sort of "S" path (tilted 45 degrees left). So the belt comes from the vertical to the left side of the black pulley under it then over and around the grooved spindle that comes out of the motor.
Hope this makes sense and helps.
Cheers
I managed to break the front dial in the process somehow though..... another £15 no doubt
andy white
January 2006