Ha, I did my sums wrong. I was only 80 quid a *year* better off, and because we had an increment in Mar only 21 quid a year better off at that point, which comes to a sum total of arrears of £28.80.
I guess it'll be a rather smaller celebration! Still, 3% cost of living next month backdated to this will still be welcome. And I skip the next point on the new scale because it would be less than I would have got on the old one at that point.