Fitted the wind deflector in an hour or so this morning without any problems. I thought that I might have to decide the position of the guides, but when I took the silver covers off I noticed that there are already fixings for the screws to fit into. So really all that is needed is to drill two holes in each cover.
I phoned my local BMW dealer on Thursday afternoon and was quoted £383.83 for the deflector and was told that I needed to repace BOTH THE BLACK AND SILVER COVERS. In the end I ordered the deflector kit late Thursday evening from Ebay for £200 + carriage and it was delivered at 8am this morning.