ok, I'm a bit stuck now. I've been drilling it out, and I've done a better job than I expected of trying to keep the hole central. I've been increasing drill bit sizes as I've gone, and have worked my way up to a 4.5mm bit. I'm finding it difficult to make progress though, feeling like not that much is happening now in the way of cutting. I'm using some HSS bits, nothing expensive, just something out of Maplin.
I've got far enough for part of the top of the bolt to come away, though the rest is still stuck.
I'm loathe to keep drilling because I feel it would be easy now for me to start drilling away the sides of the hole in the bodywork without me realising it.
What should I do next ? Should I try putting a tap through it ? If so, what size ?
And yes, I do feel like an idiot for having to ask so many questions about this