Hi Phil
It will be your local property taxes IBI which come out every year first week of June. I guess at some stage you must have set up a direct debit with them which I did a couple of years ago. Their offices are next door to Mileniun in Al Kasar where a nice helpful guy called Juan Jose can explain it all for you. Mine was €164.55 for a 3 bed in Jardins but yes changes each year.
The yearly Property Tax: "The Impuesto sobre Bienes Inmuebles" (known as IBI) is fixed by the local authorities and Catastro, depending on both the value of the land and of the property itself.
Hope this helps.
Gabe