The 'tax man' is HM Revenue and Customs. Pension credit and Universal credit is the domain of the DWP. Do they co-operate? I'll leave it to you to guess the answer!Probably a dumb question but surely the tax man knows now much people earned over the last tax year?
(In my years with the DWP in the 80s and 90s, I can vouch for the fact the the Inland Revenue refused to co-operate even on a basic level)