Whilst I'm struggling to find a link with community fees for a garage and the amount paid for medical treatment, how much was this overpayment?
We have underground garages which the owners pay community fees for and it is around 7 euro a quarter. As hugh_man says, check your minutes and find out.
And, just as an aside, there are around 125,000 Spanish people living in UK. Official numbers from UK living in Spain is around 300,000 and we know that is only about half the true amount. Add in the fact that there were 18 million British visiting Spain last year with just 2.5 million Spansih visiting UK then I think it's obvious why the discrepancy.
That and the UK health authorities are notoriously bad at claiming from other countries.