Update:
The existing shower valve is likely to be Hudson Reed, because as well as resembling a Hudson Reed, and having the same cartridge design, the tap on the sink has HR on it.
The replacement cartridge arrived, I fitted it, and it worked as expected. I got a call a few hours later saying that the shower wouldn't come on at all, so I went to test it - the water supply was on, but the cartridge wasn't letting water through. I turned the cartridge (the whole thing) 3/4 turn anti-clockwise, and suddenly all was working. Bizarre, it was like it didn't like being all the way in. I switched off the water to adjust it properly, and I was going to test it, when I noticed that the end of the cartridge (that turns the temperature between hot and cold) wasn't turning as far as it did earlier in the day (it was down to 1/4 turn or less). I turned the water on, the shower was running, but the temperature could no longer be adjusted properly, it was always too hot (which is what had happened to the original, but the original had lasted several years, not 10 minutes).
I've removed the cartridge and it has loosed again, now allowing me to turn it 3/4 turn (this is all by hand, not using grips). What could have caused the cartridge to fail so quickly, high water pressure (the shower uses a pump), high hot water temperature, or what?
Thanks
Mike