Thanks for the responses, it's good to have informed opinions!

The original stays were heavily crimped, allowing use of both 2.4" tyres abd a triple chainset. It seems that this is a weakness in the original because they are prone to cracking. Although my non drive side was fine and not cracked so maybe my riding wasn't too hard on it. Anyway, when I went in I said "I didn't want to lose much tyre clearance". He airily agreed to this as if it wasn't going to be a problem. He didn't offer me a choice of materials or explain what he was going to do and what the implications would be.

When I got the frame back he did show me the results with the wheel in place and was fairy dismissive and being British I took it home and painted it up. But when I rebuilt the bike and had a proper look myself I decided that there was nowhere near enough clearance. So I went back and had a further chat, he said 'oh I could make these thinner'. Ok fine but why didn't you do this in the first place when I said I didn't want to lose clearance? The original stays were 11mm wide, the replacement were 20mm wide so a 9mm lost on each side, which is huge. Not only that but there was very little chainset clearance, and since I'm upgrading to 1x11 I've fitted a 32t oval ring and it wouldn't even go round.

He said he could not crimp them as he didn't have the equipment to do it, but he ovalised them some more. There's a few mm more clearance, but I don't think I'll be able to run the adjustable dropouts on their shortest setting with 2.3 tyres in there. And 2.3 tyres are important because it's rocky where I live. I didn't want to have to compromise.

I'm disinclined to go back again, I'm sure he'll say that's the best he can do. So that brought me to the original question of whether or not I can crimp them myself. I'm fairly confident I could construct a jig to press a shaped piece of steel rod on the inside, but I'm not sure I could hear it up red without getting the rest of it too hot. But I'm not sure if it's a good idea from a metallurgy point of view.

I think I would.have preferred a more workable steel allowing the shape I wanted, but perhaps with a thicker walled tube to add a little weight and strength. But none of this was discussed which is a pain. And it cost me £200... ​​​​​I think he just didn't want to out much thought into the job tbh.