Thanks for your advice.
I did try bigger and bigger drill bits, going up in steps of .5 mm, until I found the one that was .5mm narrower than the one that gave the screw complete clearance (is that clear? The test holes were not drilled in the mast itself!).
I thought that .5mm undersize was just about right, but clearly not.
Is there a limit to the thickness into which one can screw a self-tapper?
I’ve since bought a pop rivetting set, but I’ve never used one of these before, so there is the opportunty for more errors …
It’s not obvious to me how you gauge the size of pop rivet to use. You need to know how thick the mast material is where you are going to put the rivet, but it is not obvious how to measure that. Or doesn’t it have to be very accurate?