in this case it would help to know that richard is in europe, thus farnell and elfa. the "best" available option from them would be the bourns 91 series, which in 25k is:
9357777 farnell
64-253-42 elfa
the 91 series is conductive plastic and generally a good quality pot. sadly the shaft is plastic, and the action feels a bit "light" for some people.
the alpha from mouser that nydave mentioned would be a better feeling choice, and it is even available with a switch to switch off the sidechain:
315-2415F-25K mouser
i wouldn't hesitate to use the alpha part for a diy build, but it may not last as many years as the bourns. in this case, the shipping cost may not be worth it either.
ed