The one that Nish suggested is good, I haven't been there but it's great price, good luck...
if not, you can try this one:
Pentax 50mm f1.4 Lens 100mm 10-17mm 12-24mm 14mm 16-45mm 18-55mm 20-35mm 21mm 28-105mm 21mm 43mm 70mm 35mm f2
It's just off Tottenham court road (that's the closeset tube too - address is on top of the page) but their asking price for that same lens is £370 but they have it in stock (according to net but it usualy doesn't lie)
Jacobs has best price £294 but they don't have the lens (though you may try in store, you may have luck - biggest branch is in New Oxford street maybe 3 min from the same tube station as above)
and there is one more shop in Totthenham court road (I'm sorry I can't remember the name but I'll be around the area tomorrow so I'll check it) they have great pentax stock but unfortunately the guy tries to rip people off, you may negotiate price a bit though, if you want to know more details about any of these or other stores, just PM me....
BR
Peter