[Arran]

 In the past I have used laundry soap, mixed with hot water, and left them soaking in that. I have also used tri sodium phosphate cleaner, also in hot water, it does not take as long as the laundry soap and you don't have to deal with as many suds. However I may try dish soap and water overnight as I have noticed that the Bakelite knobs ends up looking a little dull with the TSP hence needing a wax job afterward. The TSP does not seem to dull the finish of any other type of plastic, like with polystyrene, polyethylene, and ABS is comes up like new, the strange part is the TSP does not seem to affect Bakelite tube bases or sockets so I suspect that the manufacturers may have waxed and polished Bakelite knobs originally.
Regards
Arran