Thanks for hints and I was right. NI receipts were £75.8 billion in 2009-10 and expenditure on benefits was £75.4 billion althought with some other costs taken into account the NI fund ran a £4.6 billion deficit but that still left the NI fund £48.6 billion in credit at the end of that year. See:
Expenditure on state contributory based pensions was £66.4 billion, on incapacity benefit £6.2 billion, and on contributory Job Seekers' Allowance was £1.1 billion.
So the scheme currently runs as a pooled risk / inter-generational transfer.
Interesting and politically important.
David Byrne