Between 2003 and the end of 2006, UK citizens and other non resident EU citizens paid 35% Capital Gains Tax (CGT) when selling their property, whilst Spanish nationals only paid 15%.
The European Commission has said that this overpayment contravenes European Community Treaty discrimination rules and should not have been charged by the Spanish Tax Authorities.
Therefore, if you meet the requirements below, you are entitled to reclaim this overpayment, plus approx. 6 % interest annually from the time the CGT was originally paid (estimated average to reclaim: 20 - 30.000 €).
More legal info here