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A thicker set of T&Cs I found (that came with the annual statement) talk about "10% of opening balance" so I think I'm good.
(Will have to work out what to do for the final 5 years though, as getting a new interest-only deal would mean only being able to pay off 5 sets of the 10% each year. ERCs would add up to £3k or so. I could just convert it to a repayment mortgage then I suppose. I don't think there's such a thing as a fixed interest-only offset mortgage.)
With mine it was 10% of the total. Different bank though.