Jobs in the public sector comprise a richer mix of managerial and white collar functions than in the private sector. There is for instance a very large and poorly paid retail sector in the private sector, which has no comparison in the public sector. Counting all these wages in and coming up with a mean average is pointless. What you want for the purposes of your argument is a like for like comparison. Policy Exchange, a right leaning think tank, have sought to do this. Here is a study which, although controversial, makes the case for what you're trying to say more effectively http://www.policyexchange.org.uk/publications/publication.cgi?id=240