A soaking in Dettol and scrubbing with a toothbrush stripped most of the paint, some thinners applied with a cotton bud took car of the residue (pictured top left before I used the thinners).
Re-primed it all last night.
I've applied a few light coats of yellow (from Halfords this time) this morning, and am working away this week so that should give the coats time to harden before I wet sand them to get any dribbles out, hopefully it'll just be a couple of coats after that!
After some advice from Oly, and some reading on RMWeb, I now realise how tricky yellow can be.