Thanks for the info.
It would appear my stop start is intermittent (kicked in once on a drive, but couldn’t get it to do it again)

I also appear to have the capacitor and not the second battery.

I have a new (second hand) brake servo and master cylinder to fit and I will then do a full bleed of the braking system to eliminate any chance of air in the system.

If that then doesn’t work, then ABS pump will be the next on the list to look at.

A very niggly frustrating issue as it appears to be intermittent with starting on random occasions but has no effect on the day to day running of the car.

As there are quite a few pipes, check valves, connections etc. in the vacuum system is it worth trying to test the vacuum system for leaks in someway with a vacuum test gauge, as the stop/start valve appears to be part of that system as well, before you start changing to much, only a suggestion.

Cheers
Col

Yeah this is something I am hoping to do.
I am starting with the vacuum pipe which goes from the servo to the engine, as this does look to contain some sort of inline pressure sensor.

So will probably be a case of ticking potential causes off the list as I go along.