I have just finished looking up Delrin on google UK and came up with the same supplier. It's cheap stuff but available only in sheet or rod not for example square section. Not easy to route out a piece of rod.
This site seems to be the biz for enclosed track guns.
Internet Custom Spearguns Yank of course but a good read. Gives good techy detail.
There was a good semi enclosed home made wooden gun on medfish recently but tolerances are large for wood, whereas plastic works on a very little, like only 15 thou' (inches).
Those custom guns are mean SOB's as they should for $1000 and check out the claimed ranges.
Dave
Stop Press. Just checked the Delrin site again. Looks like you could get sheet cut to size to make square sections. 1ooomm x 20 mm sq is only £6 although there is a min £25 order charge and maybe a cutting fee plus postage. However for say £30 looks like you could get enough of the wonder plastic to make half a dozen tracks.