We have used Spanish Tax Forms Ltd for the past three years. 1st class service and inexpensive compared with the Spanish legal people. There is only a single charge from them for a house, no matter how many people own it Spanish tend to charge per person €50 - 80 each).
Once you have your IBI charge, pass on the info to them, this tends to reduce the overall Tax cost considerably.
If you need their details, drop me a line.
Derek