Front derailleur won't stay in big ring - it just springs back

oh that's what's sheared off!!

will need drilling out, if you drill the inside of the stuck in bolt the outer edge of it will come to bits hopefully saving the thread of the frame braze on

Yeah. Observe:

Did you know a NOS pair of these costs £60? Cause I didn't. I mean, I don't need a new pair, just to sort that bolt out, but I thought I'd check it out. Anyway, is it possible to get at that bolt from the other side of the frame?