By 'cashing in' your private pension you've triggered the Money Purchase Annual Allowance(MPAA), which means amongst other things, that you have to tell any other DC pension schemes you're currently paying into.
As a RM employee, far as I know that will only apply if you're paying into AVC's(Bonusplan & Flexiplan) or the RM Defined Contribution Plan, and not the DBCBS as that's Defined Benefit, or if your private pension was worth less than £10,000.
Failure to tell them when you should, could result in a fine.
More info:https://www.moneyadviceservice.org.uk/e ... -allowance https://www.pru.co.uk/pdf/GENM282401.pdf
My advice is to contact both the RMPP and the RMSPS and ask them to clarify.
Their contact details can be found here:
RMPP: https://www.royalmailpensionplan.co.uk/ ... l-contacts