I thought the battery earth to chassis braided strap looked a bit puny but then I'm used to bigger and more modern battery setups. The chassis stud the earth connected to was pretty corroded and I don't think it would survive an attempt to undo it. It worked well enough for the test I did.

I couldn't get under the car to comment on the gearbox earth. I couldn't see any earth strap around the starter motor area but the advice to run an earth from the upper starter mounting bolt to the chassis seemed a good idea.

Of course I forgot to check the size of the bolts - 17mm ?

Getting ready made cables seems somewhat difficult so I thought I'd invest in a crimping tool (Draper 64335 Crimping Tool) and make my own.

What size cable would you suggest? 16 or 25mm ? Since this is an additional earth I thought the 16 would be sufficient.

I intend to put a longer (M10?) bolt through the starter/bellhousing and connect a right-angle ring terminal onto the end with a locknut to secure it. The other end would be an M8 ring terminal secured to the chassis brace below the bulkhead.