Simon,

Go here:
http://www.americanvalve.com/flexcon...couplings.html

See if you can source something similar in the UK - any hardware/home store (if you all have those) should have it, or a plumbing supply.

If you look where the clamps seat, the coupling actually has a recessed area. I'm not sure if the recessed area is the right thickness, or the wider area outside the clamps. Either way, this piece cost $5u.s. or less

I used the 2"x2" coupler, and it stretches to fit VERY snug. Snug enough to where you use something to help it slide on, and possibly even heat it a bit to make it more flexible.

Hope this helps.

SQ