Just a quick update on how I resolved my trip reset issues – despite numerous attempts to unscrew the reset I ended up having to destroy it to get the instrument console apart. In the end I supported the end of the reset in a clamp and drilled it out with progressively larger drill bits in 0.5mm increments.
I then cut away the remainder with a pair of sidecutters to enable removal of the instrument cowling.
Even with the speedometer removed, I was still unable to unscrew the reset so ended up cutting it away with a sharp knife, doing my best to avoid damaging the thread.
This revealed the root cause of my problems – a previous owner had for some reason decided to glue the reset in place – you can see the remains of some sort of epoxy glue in the picture
The reason I had to pull the instrument housing apart was to try and fix a mis-reading tachometer – as you can see from the picture below, this was caused by grease/oil migrating from the tachometer cable into the magnet/cup mechanism that is connected to the tachometer needle. Root cause traced to a replacement tachometer cable about 6 months ago that came pre-greased/oiled. I didn’t register it as a problem at the time but I would advise anyone who has to replace their tachometer cable with an aftermarket cable to clean and degrease it before fitment. Lesson now learned!
I bought a new reset from Germany and all back together and all instruments now working after sorting out a broken wire to the fuel gauge sender unit under the tank.