I've just had the contents of my final pay slip explained to me.
It turns out that the only month to leave your employment and not get stiffed, is February (but only on non-leap years). Any other month, July as a particularly relevant example, and you get paid less per day than normal. Apparently they count the number of days you don't work and take that away from your monthly amount, and the pittance that's left is all yours.
How fair is that? Surely if I work 4 days, I should get paid for 4 days and not for 2 days, 4 hours and 40 minutes. I'm thinking of taking 3 hour breaks all next week to even it out. Although I have my payslip and my P45 and the money's in the bank ... I wonder where my incentive to actually turn up next week might be?!