We would have been worried but for the fact we had been paying the bills without fail and on time.
We took the demand to our fiscal rep. over there and it was confirmed we HAD NOT been paying the bills on our place........but on another property "we owned", well that was news to us!!
After delving deeper, in a nutshell a b0llock had been dropped when houses and land had been combined to one number a few years back apparently. We were still paying on some old number but not the new one, yep it baffled us too

So, the upshot was we owed years of IBI on the house, no way apparently could the payments we'd made on the "ghost" house be transferred to the real house.
It appeared we had to settle this "debt" and claim back the bogus payments...........so for an easy hassle free life we stumped up, that was all me holiday spends gone on the first day of that holiday

I then became annoyed I'd stumped up as I soon realised getting the money back would be like getting blood from a stone, which it was. Every year we'd contact the R de P office only to be told that finances for that year had been used up........maybe next year.
So imagine my delight yesterday when I receive a letter from the Patronato de Recaudación saying that they have issued instructions to the treasury office for immediate repayment of our money..........plus I was gobsmacked they're paying it back with the accrued interest over the time they've had it.
OK it was a right royal PITA to have this happen but at least they came good in the end. We'd given up on it to be honest, so it'll be nice to have a "free" holiday on our next visit.
Can't help feeling it'd have been easier to sort out in England though had the same thing happened.