Thank you everyone. Here are some photos of what's left of the groupset (plus a couple of random bits) for anyone who is interested. I guess there's no guarantee this is what originally came with the frame, but it's what arrived when it was given to me.

I'm now convinced it's not a Hetchins. No huge loss, I still really like the frame. Money is fairly tight at the moment so I'll sit on it for a few days before deciding whether or not I can afford a new project! And I'll poke around a few more websites to see if the serial number can identify the frame. I'll update the thread if I confirm anything!

Mafac calipers
Mafac levers
Sachs-Huret rear derailleur
Huret front derailleur