Symptoms

It started making an unusual noise, then stopped drying altogether. Clothes were warm, but had only dried a bit on one side. So the heater is working, but was the drum turning? Throwing a spoon in the drum and switching it on, there was no sound of the spoon clanking. Opening the dryer, the drum could be turned easily.

Conclusion - the drive belt is broken. I ordered a pair of replacements. One to fit now and another to be ready to fit in another 5 years.

When they arrived, I opened the dryer and found the tension pulley had broken. The plastic wheel had come off the metal bearing. That would explain the unusual noise. So I ordered a pair of those. 

My dryer was not identical to the one in the explanatory video. I could not get the front off. Eventually I managed to slip the drive belt through the tin gap between the front and the drum. The hardest bit was slipping it past the two rollers that support the front of the drum.

There are two more hurdles to overcome.

Firstly, the motor spindle goes straight to a fan at the front. I now have to find how to get the belt past that. The fan has a hexagonal nut shape, suggesting it could be screwed off the spindle. But I don't have a tool that can grip the fan nut where it is.

Secondly, the fan housing is in the way of removing the C-ring that holds the pulley wheel on its axle. This will be tricky because I don't have a tool to grip the holes in the C-ring.

I looked for the service manual for my dryer, a Bosch WTE84106GB/48. Google returned this:
https://www.bosch-home.co.uk/customer-service/WTE84106GB/48#/Tabs=section-manuals/Togglebox=accessories/Togglebox=manuals/Togglebox=spareparts/

Alas they want proof of being a professional repairer before they will provide information about how to dismantle or repair stuff.