Where is the reg that states all income should be taken from the same tax year ?

I went to a tribunal and to cut a long story short the tribunal judge directed that - for one particular annual review calculation - that I should be assessed on salaried income from one tax year, together with my additional pension contributions from a different year (which he ruled counted as additional income for assessment under the 'diversion of income' provisions).

Made up figures to illustrate what I mean :-

Tax Year    Taxable Salary income     Additional Pension Contributions

2019-20     £30,000                                         £5,000

2020-21     £25,000                                         £10,000

The most recent tax return available at the time of the annual review (in November 2021) was for 2019-20, however the lower tribunal judge ruled that I be assessed on £40,000 - i.e. £30,000 salary from 2019-20 plus £10,000 additional pension contributions from 2020-21

I have asked the Upper Tribunal for permission to appeal, arguing that the Lower Tribunal judge has made an 'error of law' in this regard, however the Upper Tribunal have refused me permission to appeal, saying that it is not clear that I can demonstrate that an error of law was made.

For info, I also argued that because these additional contributions were funded by 'salary sacrifice' - i.e. where the higher the additional pension contributions, the lower the taxable salary - then it is also highly 'inappropriate' and 'unfair' to take these two figures which are so closely related from different tax years. But of course fairness doesn't come in to it - I have to show that an error of law was made.

Any pointers to the regulation that states all income figures should be taken from the same tax year ?