That's easy to answer. If you live in Spain for more than 183 days per year, you are deemed to be domiciled and tax resident in Spain. That therefore means that ALL your worldwide income must be declared for tax. A big however - if you pay tax on any income outside of Spain and that country has a Double Taxation Relief agreement (DTR), then your Spanish tax liability will be reduced by the amount of tax paid elsewhere. The UK does have a DTR agreement with Spain. I understand that Civil Service pensions from the UK are taxed in the UK and paid net of tax. So, although you would have to declare the total income from those payments, you would not be taxed again by the Spanish Tax authorities on those amounts.
It's not an either here or there decision. You are liable to pay tax in Spain.

Just to add to my previous post - if you talk to HMRC at Newcastle (details from their website) you can complete forms that will get your income given a coding of NT (No Tax deducted), so save the problem of reclaiming each year. It's not a quick process but relatively easy to do and worth it in the long run.

thanks David. To be honest I would rather have preferred to continue paying my pension tax in the Uk but if I can't then so be it. Thanks for the follow up I will contact them immediately as we are due to emigrate in 3 weeks so best get this sorted

And the good news is that I do have a government pension so I can continue paying my tax in the UK, just5gotta sort out now what Spain will want me to do. Thanks