Just to close this thread off, said I'd post pictures of the finished article so here we go.
In the end it was a lot of fuss about nothing (cutting a stainless steel cooker hood chimney). I had to be careful not to scratch it as I worked on it, but liberal use of masking tape sorted that. It was a lot thinner than my off-cut - I reckon it's about 0.5mm (my offcut was 1mm thick) so the main issue wasn't so much the overheating and the cutting, it was stopping it flapping about while I did so! Jigsaw on slowest setting, really fine-toothed blade, timber offcuts to clamp it. All good.
Also, because of where the notch ended up, I didn't have to make the arch, I just had to cut a couple of straight lines. I'm sure I'll find other uses for the 25mm punch!
Here's the finished job:
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The feed was from the adjacent downstairs WC, where the service cupboard is located:
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Someone was asking what a "distressed look" metal-clad socket is... it turns out it's a regular Contactum metal-clad socket, galvanised:
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The client bought them from
here. So, £39 for each socket, (including £14 back box).
I was quite pleased with the job, because the client preferred my old tarnished conduit/saddles etc that had been sat in the shed for a while, so it was nice to get rid of some old stock!