The stall I am thinking of in particular is run by a middle-agedish couple, and is just down from where the road crosses in the middle of the market, towards said P&M shop. Of course there are others, like the shady-looking pair selling ragga, dance-hall etc. it may all have changed, I haven't been there for a couple of years.
If you're in my neck of the woods, David's Music in Letchworth is good. Today, for 4 quid, I got a McCoy Tyner compilation, and Roy Haynes 'Cymbalism' (those crazy jazz drummers and their album titles). I'd only gone in on the offchance of getting that Lou Donaldson. Oh well. There's also a stall in Stevenage indoor market that can throw up the odd goodun, too.
Cheers, Bob