Was sent a link to this site, which some of you stats nerds may also find interesting!! http://www.howmanyleft.co.uk/
This site takes the DVLA licensing data and aggregates by model it to tell you how many of those vehicles are being licensed each year.
While it's not the most accurate way, it gives a good indication of how many of that type of vehicle are out there.
For example, you can search for Ducati: http://www.howmanyleft.co.uk/?q=ducati
and see how many of each are being licenced or SORNed.
Ducati's Flagship 1198R Corse edition - 13 models!
Some interesting stats in there.