Thanks again Milo.
I have got the manual pdf link - one of those you have to download every time - but didn't spot the relevant bit in time available.
Roof leaking, irrigation system not squirting, mini-van still missing, son's back yard flooded and tortoise needing mouth to mouth - How the heck is this retirement?
So, hopefully, the capacitor will revive after a charged battery is left in for a prolonged period as I don't have the external power supply unit.
Back to misfire problem. Valve lifters the next suspect item I'm told.