Well, I'm sure I could work it out, given enough time, but it looked like a long time and I would have to buy tools as well.

So I booked an appliance service man to fix it, but chatted with him as he fixed it. The first thing he did was say it was much easier to take the back off. So he did. I watched him repair the dryer, and noted what he did so I can do the same in another 5 years time.

Remove worktop (230) and sides (201,202).
Remove heater hood and seal (204 and 205).
Remove drum bearing (231)
Remove back panel (209)
Remove drum (407)
Remove front fan lid (526)
Loosen bolts holding the motor (502) down.

Lift motor to slide the new drive belt (402) over the front fan (506) and onto the grooved motor axle.
Replace old tension pulley (504)

Tighten bolts holding the motor (502) down.
Restore front fan lid (526)
Restore drum (407)
Restore drum bearing (231)
Restore back panel (209)
Restore heater hood and seal (204 and 205).
Restore top (230) and sides (201,202).

Note there are several types of screw.

Fine-thread screws join metal to metal.
Coarse-thread screws join metal to plastic.
One countersunk screw fastens the plastic front fascia (107) to the metal frame.

He told me he had the same dryer as I did, so that's a good recommendation.

I hope this helps other people who want to do the same maintenance job.