Thanks Firepower. Nice to hear your thoughts

I find the whole rom / prom changing story confusing and Clay's guide whilst great and very generously provided would benefit from some quality editing to make them more readable and usable. So much info is repeated and placed in ad hoc locations. 

Agree 100%. But they are still an extremely valuable resource. 

The LED that has been added to your driver board is a blanking signal indicator by the look of it - do you find it any use?

Yes. I actually installed it whilst trying to get the CPU running and found it a good visual indicator as to what was going on.  I guess I was used to a blanking led from my last system 11 repair.

As a thought what about a super cap and removal of D17 for the battery replacement?  About the same cost as batteries and a long time maybe more elegant solution.  It seems popular in the Bally world, but not in WMS which I don't understand.

I do like the idea of keeping the board nice and tidy.  Will definitely look into this for future.  Are there any drawbacks to these capacitors?
I know that you have to power them up regularly to maintain the charge but that's hardly a drawback.