I have done a similar sort of thing recently.
The procedure was that a contract/document/power of attorney in Spanish and English was sent by my abogado to the UK.
It was taken by the signatory to a Notary (You'll find one on the Notaries Society web site), signed in his presence, witnessed and stamped. - Cost around £50
The signatory then took it to the Foreign and Commonwealth Office in Milton Keynes and had it 'notarised' with an Apostille attached.(This part can be done by post but takes longer - personal visit done same day) Cost about £30 I think, If you go on the F & C Office web site it gives you full information.
The completed document was then returned to Spain 'signed for postage/courier'
Job Done
Sounds complicated but was fairly easy.
Hope that helps