Audiocontrol Epicenter series 2 bass control?

I don't have any schematic info, but Audio Control still sells spare parts for these as I recall. I suggest you google search their web site. There you will find loads of info on their products and including spare parts for sale like the control knob your asking about.
They also incorporate a switch inside the pot to turn off the epi=center as I recall so no its not just a pot but a pot with a pull switch. Your in England so shipping will be the major pain. But AC backs their products very well so I am sure they will sell you one.
To my knowledge AC has never released any schematics and I have been working on their products since they first started in the business. Just google their web-site and search for spare parts they sell them ready to plug and play....Hope this helps some....