When you say "over the last year" do you mean since the start of the tax year i.e. 6 April 2012? It would be impossible to work out without knowing your tax code. But if you are back at work then your employer should work out your year-to-date tax when they calculate your next salary and will make any correction needed
I'm just starting back at work having taken almost exactly a year off. Over the last year I've earned around £10 000 and paid £590 tax. I've tried to do the sums, but I just can't work out if I've paid too much or not - and if I have, what do I do about it?