Thanks to all who suggested various fixes for my starting problems last year.
Eventually I found that Both
ignition coils were suspect. Off-load secondary coil resistance checks didn't bear any resemblance to those specified in the Haynes manual. The primary windings appeared to be in-spec.
I purchased a new pair of coils last month and after fitting them the bike started up immediately as it should with no delay or prolonged turning of the starter motor. Now just waiting for good weather to give it a blast.
Still can't see why both coils failed at the same time.