So, I've only gone and done it!
With a bit of help from somersetstroller...
I thought that was going to be a long standing battle, the kind of thing that finally tips a man over the edge... but... solved!
I can't see a broken clip as such, but the little bit of laxity/play in the plastic/rubber trim at the front of the roof rail was the issue (see below). Wind obviously getting under it. A bit of unsightly parcel tape confirmed the issue; now I need to bodge together a less ugly solution. I've got some 2mm rubber strip, so I'll probably shim it tight with that.
Thanks all! And to Regulo for that LOL
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