I would not know that unfortunatly, but I am sure there is a reasonable explanation. If you really wanted to know, the best thing would be to email the Met Office?
Also, I think I remember reading somewhere that in the earliest days of the CET, temperature readings were taken indoors because it was too cold to go outside. If that is true (a very big if, I know), then that would be a much bigger problem than the changes over the last 5 years.