OK, finally have something to report on this.

I replaced all the components listed above which the Bally repair manual said were likely causes of the LED locking on. Unfortunately this did not fix the problem. When I visited Benpin the other day he mentioned that I should try replacing the 6821 chip at U11. So the very last thing I tried before I was going to send the board away was to switch U7 and U11 (as I didn't have any 6821s lying around). Well, now instead of a locked on LED, I have one flash. So I'm guessing one or both or these 6821s must be faulty.

So it was very frustrating that I changed out all those other components for no reason but good to have made some progress.