Why won't my oven heat up?
I appreciate everyone's help with this, but I'm posting an answer myself a I'm the only person who can in this case.

The final cause of the oven not heating was a broken selector switch. See image.



With a new fan element, thermolimiter AND switch - the oven is heating up fully on all settings. Spare parts cost about £90.

I think I may have a further issue with the ventilator, but I'll post a new question about that if need be.