tax on UK pensions

Hi, I'm looking to move to PDC permanently in Jan next year (and purchase property etc) and pretty much up to speed on residency requirements and processes ( am British but hold Irish/EU passport)

My question really is with regard to Spanish taxes on UK pensions (not state). I have a civil service pension and a post office one which are both taxed in UK.  Obviously UK/Spain have a DTA so I wont get taxed twice but am I obliged to inform Spanish authorities and pay this tax/taxes in Spain and then get credited by HMRC or simply crack on?

Any steer would be appreciated or point me in direction of local advisor I can approach etc.

Thanks.....

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