cheers beejam! (STW really does appear to be the fountain of all knowledge!)
here's what I've just gathered, if it helps you?
register at town hall in madrid (but two years regestration is required, without that you need "According to documents as seen" from the BC, see below)
GET - full UK hague Apostille on birth certificate
GET - above document translated - by the ministry foreign affairs: http://www.maec.es (?)
GET - Proof of UK residency for +2 years (household bills etc)
GET - Non Standard document from the British Consulate known as "According to documents as seen" (80 euros - approx)
GET - UK town hall to do the 21 days publication of Banns
GET - Banns translated by the British consulate - (75 euro approx)
All documents only valid for 3 months from UK issue in Spain (regardless of UK or international document status!)
Paseo de recoletos
nearest tube: banko de espania
opening times: 8:30-13:30 mon-fri
I spoke to a really helpful lass called Bernadette at the B.C.