I'm no expert on this but my feeling is Leroy Merlin are 'trying it on'. I would have thought consumer rights are pretty similar throughout Europe as we are all dealing with multi national companies  these days. There is no doubt that in the uk the shop you bought the pump from would be liable to either re-embuse you, fix the pump on your behalf or give you a refund.
I think you should go back to Leroy Merlin and insist that they provide you with the name and contact details of the store manager. You can then write to him in Spanish ( maybe you know someone who can help with this) and make a formal complaint. Find out what the equivalent of 'trading standards' over there and tell the manager that you will be contacting them if the matter is not resolved.
Good luck.