This week they published a standard note entitled 'Absence of a Prime Minister at Prime Minister’s Question Time', listing every occasion at which the Prime Minister of the day missed PMQs (No reasons given - but I'm sure there were good 'excuses' - attending summits, handling crises etc). Interestingly the percentage of missed PMQs are:
Mrs Thatcher 7.7%
John Major 11.8%
Tony Blair 5.0%
The full paper is available at http://www.parliament.uk/commons/lib/research/notes/snpc-04401.pdf