Re: 75/400 Creda tumble dryer (bought 1978)
Have you removed the pulley pin and threaded the belt through the pulley assembly???
It looks as though you have from that picture, you should not have done that, the pulley should have been left untouched.
The grooved side of the belt should run on the drum and in the grooves on the motor shaft. You needed to unbolt the fan from the motor shaft to allow the belt to be slid onto the motor pulley and then the jockey pulley should press against the side of the belt as shown in Martins diagram.