There's a reclaimation yard in Trumpeter's Way, which is just off the Boston Manor Road (I think it's that one, anyway) in Ealing.
They have a fair range of stock in, which is important because you'll want to match the colour and size fairly closely. Price - variable. Quality - rough and smooth, kind of bought as seen, buyer beware, all of that stuff. Don't expect a pristine B&Q-type orderly place though...!
I'm assuiming it's yer standard Victorian-terrace-converted-to-a-flat? If so, then brand new boards are likely to stick out like a sore thumb.
Oh, and if you're upstairs, just check that your lease doesn't specifically demand that you maintain carpeted flooring. It'd be a shame to do all that work and be forced to cover it by the freeholder.
cheers Richard
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk