Open Question: What proportion of existing mortgages are lifetime trackers in the UK?

I heard that about 40% of existing mortgages in 2009 were trackers, but I understand that a lot of trackers are only for fixed periods (say, for 2 years) and they revert to variable rates afterwards. So, does anyone know what proportion of all existing mortgages are lifetime trackers in the UK? Also, what proportion are lifetime trackers with no 'collar' (ie. no interest rate 'floor', so they are really benefitting from the current low rates). Anymore details such as the typical interest rate of such trackers would be appreciated.