I've got this pinned down now and I'm definitely right despite what my new best friend says.
UEFA produce a 'Toolkit', which is a software model that allows clubs to submit their returns in a standard form. Along with this there is guidance about what to include and how it's calculated. The 2011 version of this toolkit talks about the conditions for being able to use the wage exemption and, according to that, we could use it based on the example used to illustrate it. Here's what it says:
(ii) It proves that the aggregate break-even deficit is only due to the annual break-even deficit of the reporting period ending in 2012 which in turn is due to contracts with players undertaken prior to 1 June 2010 (for the avoidance of doubt, all renegotiations on contracts undertaken after such date would not be taken into account).
For the avoidance of doubt, condition (a) means if the quantum of the aggregate break-even deficit that exceeds the acceptable deviation is greater than the quantum of the break-even deficit for the reporting period ending in 2012, then condition (ii) is not satisfied – because part of the break-even deficit in excess of the acceptable deviation is due to the break-even deficit in 2013 and/or 2014
If I've calculated it right, the total of the aggregate break even deficit exceeds the acceptable deviation by £76m. The break even deficit for 2012 was £83m. Therefore condition (ii) is satisfied which means we could have used the £80m exemption and escaped without sanction.
In 2013, after we'd published our 2012 accounts, they issued a new one and the guidance notes had changed to say that the 2012 wages (£80m) had to be greater than the break even deficit in 2012 (£83m) to satisfy condition (ii).
So from being able to use the £80m to pass, without changing a single figure, we suddenly found we couldn't use it and therefore failed, when it was too late to change anything. How convenient.