Hey Dawn
Lucky you!
In all of them you really need to ask them to dig stuff out for you - some are better than others. The male SA in the Harrods concession appears to be really good and will dig out stuff from through the back. Sloane Street I found the SA's a bit lacking in knowledge. To be honest they are all much like for like so I would try them all. Don't be put off with lack of stuff out - make them go through the back and dig stuff out. Also if you cant do all of them ask them to call Bond Street etc for you.
Good luck with your hunt!