So I struggled with it and Googled for solutions without much success. Then I thought maybe I should buy the £5 downloadable repair manual from Selfix, but wondered whether they would give the same apparently ineffective solution as others. But in fact, when I looked through their FAQ I found the really ingenious answer. It's here:
The belts on the WMA and WMM range of Hotpoint Washing machines are loosely classed as stretchy belts (although you would never think it to look at them). This means that there is no adjustment on the motor. To fit a new belt (or even to replace one you have just removed) is simple if gone about in the right fashion. Whilst standing at the front of the machine, with the top and back inspection panel removed, lean over the machine and hook the belt over the motor pulley then feed it onto the right side of the main pulley as far as it will go. Hold it in place with a few cable ties wrapped around it and the pulley. Put your left hand inside the drum and rotate it slowly anti clockwise. This will draw the belt onto the main pulley and the cable ties will snap as the drum rotates.
* didn't put my hand inside the drum
* cut the cable ties off with scissors once the belt had popped into place
* adjusted the positioning of the belt by hand (to make it more central) afterwards