LEAK TROUGHLINE STEREO good serviced with wooden sleeve 160
LEAK TROUGHLINE II good and working nicely 120
studio 12 decoder for leak tuners 65
LEAK TROUGHLINE III mono black and silver serviced 80
LEAK TROUGHLINE STEREO TUNER serviced 120
LEAK TROUGHLINE I metal front in gold or champaign serviced 50
STEREO DECODER new valve model for Leak Troughline, fits inside the tuner 175 skilled DIY or 200 fitted
Choice of 2 decoders there, there are others too. I believe the TDP one in your link was a design done for hifi world a few years ago, and is reputedly pretty good. Bubs got a GT audio decoder I believe which I know he rates. Ive heard the troughline with its own stereo decoder, and even that is very very good IMO...and at £100-120 is a bargain too I think
I think a 'starting bid' of £300 is taking the piss a bit, I mean whats he expecting??? £400-£500? you could get 2-3 of them for that!

Never bought one myself, I just dont like the music played on the radio much.
Chris