Stuart
This would have been Ibiul-Imp.Bienes Inm. commonly known as I.B.I. levied by local town halls as a local tax for owning a property in their municipality. Local town halls are given licence to raise taxes locally by the Spanish Govt. to pay for local services and facilities, someone has to pay for those magnificent Town Halls
For queries contact sanjavier@recauda.net but be prepared to talk Spanish
It will have been charged from the moment you completed on your property and should be approx Euros 370 per year for a 2 bed apartment but is based on the Valor Catastral roughly equivalent to a Council Tax or local rates valuation for your property, no account has been taken for the fall in property values.
You will have had an invoice sent to your registered address, probably in the UK with the Valor, the amount and instructions how to pay, you must presumably have given your bank instructions or permission to pay on your behalf. If not paid the Town Hall will immediately add a fine and chase you for payments.
This Tax is on top of the contribution you make to the Entidad which covers local services such as bin and road cleaning etc. as well as security and public area maintenance