Also at Covent Garden (see the previous post) there's a rather small Field&Trek which is about two shops up the street where Ellis Brigham (mentioned in the previous post) is on the corner - I can't at this moment think of the name of the street, begins with a M - Maiden Lane? Anyway, it's not that important, you can see the shop it's that close. Further along the same street there's a shop called Kathmandu, sells some major brands.
Also in the same area is a branch of Snow+Rock, on four floors, that's easily missed as it's in a narrow turning off Long Acre (that's the street Covent Garden tube station is in) - head past the station in the direction of Stanford's map shop in Long Acre (here you will be able to find maps and guide books for Nepal). I think the little turning is Mercer Street? It'll be on your right side before you reach Stanfords which is on your left.
There's a branch of Field&Trek on the fifth floor of Lillywhites sports shop which is on a corner at Piccadilly Circus, and in Piccadilly itself (the road of that name, that is) there's a branch of Cotswold Outdoor, on the right hand side coming from Piccadilly Circus.
Finally, for now, there's another Field&Trek in Baker Street, to the south side of Marylebone Road, and another Cotswold Outdoor at Shepherds Bush, next to the Central Line Tube station (I think this station is currently closed, no escalators, but the Hammersmith & City station is just along the road).