On investigation two out of three springs in the starter clutch were about two thirds the length of a new one and one was broken up completely. So, as well as replacing the HiVo chain and the cam chain, I bought a set of starter clutch springs and a new starter sprocket from Cradley Kawasaki. Have fitted them and everything seems ok - nice and solid. Intend to run the starter motor on the bench before putting the engine in the bike, I'll let you know how I get on.
Re the oil: apologies, I made a mistake, I used a semi-synthetic 10-40 oil called Power 1 from Castrol.
I think the reason for the spring wear and break-up in the starter clutch could have been the slack Primary Chain. I believe the chain was jumping the sprockets at low and high revs.
As a further bulletin I replaced the starter clutch springs only and the idler gear. Cradley kawasaki in the UK have these in stock. It all works pretty flawlessly now. I am using a semi-synthetic oil (Halfords own brand).