A recent UT decision is here > http://www.landstribunal.gov.uk/judg...RX-55-2011.pdf
And the winner is............*drum roll*..12 years..probably

The conclusion is that the law says its 12 years but an FTT still has the power to strike out a claim for being unreasonable if the LH waiting so long to make a claim, which is kind of fair but many LH, me included, didn't sit there twiddling their thumbs 10 years ago, its just that we were completely ignorant of our rights.
I note that they mention Section 32 of the Limitation Act, one of my personal faves now, as I used it successfully to claim back OVER 12 years of GR overpayments.