Ironically this place has the exact parts http://www.britishaudioservice.com/parts.php?mfg=1&cat=14
I say ironic because of the name, based in Nashville
Trouble is it would cost a lot with postage - $27 in total. Not my amp or money, but still if I can get the parts here it would be better. I've found the main pot on ebay from Hong Kong - quality issue? But no luck with the slider. RS, Maplins, Farnell have nothing as far as I can tell.