I checked all of the carb idle settings this evening - the top two were spot on 1.5 turns to lightly close but the bottom one was way out (nearly 3 turns to close).
I adjusted that back to 1.5 turns; throttle advance lever to fully up; pressed choke in whilst turning the ignition key and it started second time (a first for this outboard).
I ran (with a hose) for about 5 minutes to warm up and then stopped for a couple of minutes. Starting warm without any choke but throttle advance on, worked every time I tried.
So it looks like a combination of user stupidity (used to Yamaha starting not Mariner) and an idle screw was all it needed
- so many thanks for all of the suggestions.
I'm going to get a spark meter and new plugs just in case however - another good suggestion !