Thanks for your question, I am Sam and I am one of the UK tax experts here on Just Answer.
Payments are usualy treated as being liable to tax on the date they are amde, rather from when theyr are due.
But the key factor is , is that you advsie this was an advance on the first pension payment rather than the first pension payment and this changes the position, to one that says, this gesture on the pension providers behalf, does not then form a tax point on which deuctions should be made, but recognises that this is merely an advance on the payment becoming due.
Whilst there are special rules for authors and the like with advance payments, this does not fall into that remit of being given special treatment, in the samew way an advance of wages also does not form a tax point (as the tax still becomes treated as arising on the official pay day rather than the advance payment day!)
I am sorry to not be able to offer the advsie youw ant to hear, and I am sure the pension dpet are equally frustrated by the matter - as I am sure they would not wish you to suffer more tax than you need to, but they are right on this matter.
If you still in dispute then do write to HMRC who can provide you with an official determination on this matter. You can write to them c/o the address on any correspondance you have received fom HMRC in the last 12 months (they change postal address frequently) but if you would like the latest address its
Pay As You Earn
HM Revenue and Customs
Do let me know if I can be of any furtehr assistance