Thanks every one. I'm now the proud owner of a new brake switch.
I live in Sussex but work in cental London so hence my request for central London MB dealers. I ended up using Hampstead (which I must say doesn't seem to have the sharpest/most welcoming of staff).
Now all that is left is for me to fit the new switch tonight when I get home. Hopefully that will solve the problem of my brake lights staying on.
Upon checking that I had the right part (I had the old switch with me) I noticed hat the rod that gets pushed in was actually shorter than the new one leading me to believe that old one hadn't fully retracted hence the lights think they are on. Any way... fingers crossed!