Pipe weld is OK, of the type plumbers use for plastic waste pipes.
Excellent, I had not considered that, and even better, have some already!
I did think about old "airfix glue" - but thats rare these days it seems!
I assume the pipe weld melts it, so will be very strong.
I think I will use evostick for the valve, just incase I ever have to remove it...
BTW - how do I test the valve? - if I gently blow in the wee side entrance, (ooh err, missus).. I can see the valve opening/closing - is that a good enough test or do I need to check its airtight somehow?