Endeavour Overvoltage Problems

Sounds nasty. Overvoltage can fry things quick. Seeing that many faults points to a real issue. Have you checked the battery's resting voltage and the alternator's output? An overcharging alternator is a prime suspect. Also, any recent work done on the electrical system?

I've not checked the alternator output, where would I measure that, and what should the voltage be? The battery is pretty old, but could a duff battery cause this? No recent electrical work that I know of...

A heavily discharged or defective battery can definitely cause overvoltage issues, the alternator strains to charge it. Check the voltage across the battery terminals with the engine running. Should be around 14V. If it's way higher, the voltage regulator in the alternator is likely shot. Considering the symptoms and age, best get it to a workshop. Overvoltage can seriously damage ECUs.

Took your advice and got it to a local garage. Turns out the battery was toast, causing the alternator to go into overdrive, and it fried the regulator. New battery and alternator later, and I'm 290€ lighter, but at least it's running sweet again!