I bought an orchid from the Stoney street lady for a fiver a year ago and it survived until a few weeks ago when i replaced it purely because it had lost its flowers (but was still healthy and would have grown new flowers had i not been wanting to impress my visiting parents with my flowering plants!).
Lil, there are two florist stalls on Stoney Street, it's the one closest to the Southwark Street end (possibly No 2 or 3 Stoney Street). I suspect that it isn't the one you mentioned as the other one has more made up bouquets and is more pricey.
BTW Flick - I have the very same problem coming up, the three orchids in my bathroom are all thriving, but flowerless and the parents are coming next week, I fear a visit to the market is necessary!