"However she as good as reassured us that we would not need to take a Spanish drivers test (we are emigrating spring 2022) as agreement will have been reached between the 2 countries."
I hope you asked for that in writing!
In any case it's not just about access to the DVLA database.
Prior to Brexit other countries managed to effect exchanges based on a 'Letter of Entitlement' which was avalable on demand from DVLA, and still is.
The controlling EU directive is brief in it's requirement simply saying (my bold)-
"Where the holder of a valid national driving licence issued by a Member State has taken up normal residence in another Member State, he may request that his driving licence be
exchanged for an equivalent licence. It shall be for the Member State effecting the exchange to check for which category the licence submitted is in fact still valid"
The DVLA Letter of Entitlement has satisfied other countries in that regard and there is no defensible reason for it to not satisfy Spain.
ACCESS TO THE DVLA DATABASE IS NOT MANDATORY AND THEREFORE NOT A GENUINE REASON FOR DENYING AN EXCHANGE.