Hi Chris, thanks for that.
The rear wheel is a 19". Although the front wheel looks big in the photos it is actually 19" also. The rubber saddle is long gone, and the leather on the one I have is split all over. A new cover is definitely needed.
I've been dismantling the bike today, and there will be quite a few parts that I will have to replace. All of the common wear items are in pretty bad shape (apart from the tyres which are brand new). Both exhaust pipes are cracked, and the silencers are 30% filler (these are marked "unity spares"). Some of the cables are frayed etc, and most of the rubber badly cracked.
I think there should be an air filter box underneath the saddle but it's gone.
There is what appears to be a circular boss with four tapped holes in it on the gearbox immediately above the kickstart shaft - do you know what it is for?
This is going to be a fairly long term project. I know the bike is not "original" but I am going to completely restore it in any case.