Thanks for that, Michael; all I would say is its not common knowledge in my neck of the woods, so I must get better sources . Not an easy market for manufacturers with those numbers to share out between themselves.
The MFDB install base in UK is estimated at over 1,500, some say 2,000.
Naturally, most of these units live in London, with Manchester in 2nd place.
This is due to UK photographers being "early adopters" since the early days of the Leaf "Brick" in 1992 and effective distribution channels for all the manufacturers.
Over the last 3-4 years there are a few hundreds of units being added every year, showing an annual growth of more than 20%. This is a healthy rate at any industry standard. Especially considering the shirnking market for medium-format cameras in general.
Large format cameras and specialized view cameras (architecture) are showing a healthy growth as well, now that MF digital backs are capable of producing anything and everythig (almost).