Oh well, we all know, easy does it - or, Gently does it.
Freddy1's suggestion (BTW, thanks!) is actually the one I chose, but as a
temporary fix (and with thin wire in lieu of the elastic band) before
ordering the correct clip.
I am a tad annoyed - not only they left the exhaust pipe clip unhooked, thus
condemning me to months of weird rattling, they also installed the wrong clip
for the brake line.
Let's be positive: it's all a scheme to have me tinker with the bike (not that I
needed to be persuaded, though)...