Taken him to the garden centre, now waiting on recovery again.
Bit awkward and lumpy again first start but we'd had 2 deep frosted mornings so assumed it was that.
Fuel pressure is good unlike last time, so this is some sort of ignition issue. Often fires for a second or two but then dies at 500 to 1000rpm and won't catch again, sometimes won't fire at all. Doesn't respond to throttle when he does fire.
But when running on the way here was absolutely fine, smooth and happy, kicked down eagerly to get past bicyclists, etc.
Puzzled.com. you'd think if it was a sensor it would take him out or misfire when running, not just starting.