In reply to Oo:
We have a happy ending! A large thank you to everyone on UKC, especially jon and timlongs for the cobbler.
I do have recovery (I view it as mandatory for the series of £500 bangers I've had) so first thought was to phone them while the partner tried some string shenanigans she'd seen on YouTube - they all failed. Recovery sounded optimistic at first, but then, I was told over the phone, that no French recovery agent would come out to break into my car as it wasn't worth it. They would only recover to a garage and then order new keys. I was a bit surprised at this as I clearly explained that I was willing to have irreparable damage to my car in order to get home: so either a breakdown in comms or 8pm on Friday had struck and the drivers weren't fussed about earning a small fee. Weirdly, the breakdown people were more than happy to pay for a hire car, new train booking etc to get me home. So, the ultimate backup plan was to work out how to scrap a car in France or see if I could somehow give it away to someone willing to deal with it (a bargain if you have the time, she's a great runner!) That sounded like a potential admin blackhole.
Anyway, got up, superglued key back together to see how that looked and give it plenty of drying time. Then caught the bus to Chamonix and found the cobbler open - result! He took my key and largely ignored my broken French about how delicate the thing was. He then went off rummaging on a scavenger hut clambering around the back for a blank and returned: with no blank, and only the stubby bit of my surgically botched key reconstruction.
My face dropped in horror; his had a slight look of surprise. Cue hunt number 2 for the crucial pointy end of the key. After much rummaging around the floor and in boxes (it was essentially a search for a needle in a needle shop) it turned up in his apron pocket. He found this funnier than I did. Anyway, no proper Vauxhall/Opel blank but a Renault one that would have to do. Key cut and back on the bus. He kindly accepted the contents of my wallet which was slightly short of the required €15.
The hotel owner had left me an ominous present of a hammer, gloves, plastic sheet and tape which was very generous - his wife had looked on my Chamonix venture as a fool's errand. I think he was also insentivised to get my car out of his car park. Straight to the car and the super French key turned in the door, locks pop, shouts of joy. No need to smash a window, good beginning. I'm feeling warmer towards the slightly forgetful cobbler already.
Key turned in the ignition, car starts (much to my surprise) lasts a few seconds then dies. Ok, I now have an idea how immobilisers work, I'm prepared for this. Old key fob held next to new key, turn again, car starts and ticks over happily for 10 seconds, old key fob removed and safely stashed. Car continued purring for a minute so I reckon immobilisers are only active around the ignition moment. We pull away and happily set off for Calais; singing the praises of the cobbler as we go!
A couple of starts and stops at petrol stations along the way and all is still well. There is life in the old banger yet and my faith in old cars being easier to deal with is restored.
Thanks again for all the help and suggestions, in reply to some of them:
I thought about soldering, but with rudimentary kit wondered about a higher chance of putting more metal in and buggering the lock. It sounded like it should work without the superglue even, so that was plan B.
I'd also double checked with the recovery people that I could still take them up on their hire car option if my breaking and entering failed - which they said yes to. I was quite impressed as it's just the service bundled with a Nationwide FlexPlus account. They were very accommodating.
As for ordering new keys, I'd tried this in the UK as when I bought it it only came with one. I am apparently missing a piece of paper that means it will be expensive and long to get keys through Vauxhall. I'm going to re-explore this when I get back.
And finally, the moral of the story?
UKC may have solved your problem; but you shouldn't use your car key to open beer in the first place.
Thanks!
Oo