Ended up elbows deep in leaf blower which is strange because it's been stone reliable for about 10 years.
Carb was clean internally, like spotless, air filter not too good and degrading but hadn't entered intake.
Drained tank and filter which were both spotless.
Had good blue spark, didn't appear to be flooded or dry.
In the end the muffler was plugged. I think I should make that the first thing I check on small engines going forward 🙄