I have solved this now.
I replaced the Mk1 switches for a Mk2. Also fed the switch from nearer to the seat. Green LED
lights now work.
Interestingly, the seats are also a lot warmer. I had heard that 75 seats were not very warm, and I will agree with that having used the old switch. However, with the new switch it is comparable to my previous experiences with this sort of equipment - you do feel the warmth now a lot more.
I have also discovered that I had too much / wrong sort of grease on the battery terminals through my experiments - this will be a factor too. Also, I have a budget battery fitted, at the low end of what is needed for these cars - with the extra equipment fitted now, I will upgrade to a heavy duty one when budget permits.
Thanks for your input.
Now just have to put the car back together!