I have worked out a Summer Index using the daily CET data, rainfall stats and sunshine hours supplied by the Met Office for download
The main difference from Kevin's (apart from it not being for Manchester !) is that instead of taking the avarage max temperature, I used the difference of the number of warm days (CET max 22C or more minus the number of cool days of 16C or less)
The table only goes back to 1961 as the current rainfall daily stats only go back that far
I used "Midlands" entries for Sunshine and Rainfall for consistency with the CET
2013 came out better (just) than 2003 but not as good as 2006 as shown below