I'll look for a datasheet, roughly speaking then I could buy 0.4mm 17-4PH annealed with hardness of 1000N/mm2 and any regular cold working to 0.2mm will get the hardness to approximately 1800N/mm2 (give or take a lot). It looks like a useful rule of thumb and sounds about right. I didn't know about 17-7PH, I'll take a look.
I wish I'd seen Ulbrich's online shop a couple of months ago. Seeing that was a revelation and a pleasant change from the prices I've had recently. I will call them, there isn't any PH stainless in the online shop but the prices give me confidence a special order wouldn't be too expensive.
Thanks.
Ant.