In reply to girlymonkey:
People really shouldn't fixate on NI, it's not set aside for specific purposes, it's just part of the overall tax take. SSP would be impossible to administer for self employed people - it's hard enough for employed where you have full PAYE records.
Self employment is generally acknowledged to be financially advantageous for all sorts of reasons, from being able to claim a wide range of expenses to only having to pay tax twice a year. (And I won't of course mention the 'cash in hand' jobs, though every poster here who has ever had a job done for cash is guilty of tax evasion).
I've been self employed off and on for many years, it seemed fair enough to me, I appreciated the freedom but I was certainly always conscious that it could come to an abrupt end - I could break my leg, for example - and always had that at the back of my mind.