Income Tax & NI contribution when moving abroad (Spain)

Hi, I have a question and I am getting conflicting and contradicting advise when researching this. - I and my wife are UK residents and we will move in February 2025 permanently to Spain (we move voluntarily, employer is not sending us) We are currently both employed by UK entities. - My wife will continue to work for her UK employer in Spain (they have no legal entity in Spain) and I am trying to understand what forms we need to complete. My understanding is P85 to notify she is moving abroad, and indicate on this P85 she will continue to work in Spain for her UK employer but remotely, which should trigger at "NT" code to her employer. She will then need to file locally in Spain her tax return. What is not not clear is around NI contributions. I have been told conflicting messsage about submitting an CF83 form, but others have advise me a CA3822 needs to be done instead (and her UK employer a CA3821) and completion of this should get her an A1 certificate. So am somewhat confused as to what form/process/order is correct. Separately I have a question on voluntarily NI contributions, Based on my employment record in last 20+ years in UK and NI contributions, I need to pay 4 more years of NI contributions to be eligble for full state pension at retirement age. I have been advised I can effectively continue to top up/pay NI contributions voluntarity for 4 years so that I will qualify for full state pension rather than a proportial payment. What is the process for this in terms of application/forms? Thank for your help on above questions?

Hi,
You and your wife will need to write in to get a liability decision, to determine whether you are UK liable or liable to pay social security into the Spanish system.
To get a liability decision you and your wife need to write to us at:
PT Operations North East England
HM Revenue and Customs,
BX9 1AN
Answering the following:
What is your full name?
What is your NINO?
What is your date of birth?
What date did you leave the UK?
What is your most recent UK address?
What is your abroad address?
What type of work do you do?
What is your company name and address - both UK and abroad?
Where is your contract held?
How often do you work in the UK?
What is your normal working pattern (the days weeks or months you regularly work)?
Thank you.