I've made some good progress with the S3 - Replaced the mesh for the air filter, fuel pipe and stripped the starter motor cleaned and re - greased relevant parts and now it's functioning perfectly.
Also repaired the seat hinge for flip up mudguard
I changed the oil and filter and the sump was pretty clean indicating a good service record.
Took the bike for a quick spin to warm up the gearbox and drive box oil (ready to change) albeit very cautiously, as the workshop is surrounded by farms etc, the roads were damp. muddy and the tyres are 22+ years old. The bike felt great and so decided to take it that little bit further to a stretch of road that has a very long, relatively traffic free straight stretch. Sure enough it just confirmed how good the bike feels and the engine just wants to go like a dog on a leash! I guess that the short stroke motor maybe helps but I'm wondering if it has something like a P3 cam fitted? If I can get my hands on the Classic Bike article on this particular bike then maybe that would reveal such details?
The owner of the bikes is suggesting that we wait until the end of Jan / Feb to advertise them unless anybody makes a reasonable offer in the meantime.