In the end I had the rifle rebarelled, and hey presto, no more keyholing. I'm not sure the barrel was bedded properly though, as I've had to do a bit of messing about with the sights to get it to shoot more or less where I want it (and it's still not great) - but at least the holes it makes are nice and round now.
Also, the term "Bollocks" is well known in Australia/NZ, I can assure you - I've certainly deployed it enough myself in frustrating situations!