Update. Much more cursing. The bolts were thoroughly seized and no amount of increasingly severe application of force and tools could get them to shift. Quite by accident, I oversprayed CRC onto the rubber clamping part and the whole thing just slid right off the bar:

I was able to look at them a little more closely once they were off, and the combined nut-bolt-washer thing holding them on must be at least 25mm high, and entirely rust welded to the bolt. So no surprise it wasn't in the mood for turning!

It seems you can get replacements for the rusty parts. However I would have to get the old ones off first and (having tried!) there is no way to get a grinder to cut the head or nut off the bolt without destroying everything around it. So some very painstaking work with a hacksaw would seem to be necessary.

I'm kind of inclined just to buy new unless anyone has any other bright ideas?