As requested for WUWT here is a plot of linear curve fit to plots of the same data referenced to 30 year periods from 1931 to 1995
eg. base periods
1931 to 1961
1945 to 1975
1995 to 2012* THIS DOES NOT GIVE SUFFICIENT YEARS BUT IS PLOTTED.
Only stations returning over 15 reference base years were used as noted on the normal plot. The stations in this data set are from the UK
So it looks as if the slope changes by over 10% but 1961 to 1991 gives one of the lower slopes. Choosing 1931 the slope (deg C per year) is near the maximum!