Well, my one way valves finally turned up.... they had been sent RM48 not 24 anyway, but still nearly a week.
I bought three "10mm / 3/8 inch" aluminium one way valves. The EVAP side on the valve pulled off nice and easy, but the manifold side did not want to come off, it seemed to have melted onto the plastic valve and in the process shrunk on. There is a plastic inner layer on the pipe. That seemed to have separated from the rubber outer too.
Thinking about it, this makes sense. When the one way valve fails, the pipe gets exposed to inlet manifold temperature air, because it is flowing when the valve should be stopping it. I could not remove the old one way valve so cut the nozzle part off and drilled out the remainer until I had a clean pipe.
The one way valve was a reasonably snug fit on there, and the blow test in both directions had the right result,, so things were looking good. On the other side however, the pipe was larger and the valve just fell out.
I had been unsure whether to buy 12mm or 10mm valves. Next time I'll buy a 12mm, but for this repair I have simply used Sikaflex around the nozzle to fill the gap in the pipe and provide the seal. Not ideal, but I believe it will work - Sikaflex is resilient stuff.
That's going to need a day to go off properly, and then it will be time for a pipe swap
I'll put a picture up later