Hobbsy and hsukaria:
Bingo! T'was the airbleed screw as Hobbsy so kindly suggested (it was run in a bit too far). It's starting and running just like it used to so will have to make sure that screw stays in place.
My apologies for inadvertently bringing up a past point of contention concerning fuel lubricants. I didn't follow the discussion, but I bet it wasn't nearly as flame-ridden as the controversy over cork closures versus screwcap closures for wine bottles that took place over on the wine enthusiast boards!
Thanks to both of you gents!
Davey...