I've just been sorting my post out, and my P45 from my old job was in amongst it.
As far as I'm concerned I handed in my notice on the 1st of Jan, and (had I been at work - I was actually on Mat leave) my last day/night would have been the 31/01 - and that's what I've told the IS people for my application for it........
My P45 says my last date of work was the 29/02...