Step drills arrived today from Amazon:

I used some parallels to extend the jaws upwards, and put masking tape on the surfaces to give some extra grip, then re-aligned the pilot holes:

The drain cock holes were less than 1mm from the edge, but the drills worked really well:

Exhaust hole wasn’t a problem, although the metal did flex a bit:

I ended up spraying it satin black, then cleaned up the edges with wet & dry:

I feel lke I dodged a couple of bullets with this, becasue of a) cheap tools, and b) the holes ending up so close to the edge. I can't even remember if I accounted for the clearance hole around the drain cocks when I drilled the holes in the casting…Next time I'd move them a millimetre or so further away.
Anyway thanks for all teh advice everyone, job done.