Agencia Tributaria nightmare

Hi,

I used to live in Spain but moved back to the UK 7 years ago. I didn't know about completing a modelo to say that I had moved. As far as I was concerned, I had paid all my taxes, was no longer working and earning in Spain and didn't need to complete any further Declaracion de la Renta. Since I notified them last year and completed the modelo, it has been a nightmare!  Despite sending a letter from the HMRC to show that I have been living and paying taxes in the UK since 2017, the Agencia claim that I owe them over 1000 euros. Letters have gone back and forth as it is not possible either to speak with anyone on the telephone or for me to go to an office in person. I haven't heard back from them in months and then this weekend I received a letter mentioning a "Boletin Oficial del Estado". Their previous letters came via recorded delivery but this one was not so I'm not sure how official it is or what the boletin is.

Any advice or information would be appreciated, thank you!