Trying to pay voluntary NI contributions to enhance pension but my record appears incorrect

Hello - I currently live and work in Ireland as an employee full time since returning from UK in 1990. I worked in the UK for 3.5 years in late 80s. I was an employee there at first for approx. 6 mths and then worked in the construction sector on a contractor basis. I was paid hourly/daily and tax & NI was deducted at source. I applied in 2022 to the DWP in Newcastle to check for state pension eligibility and received a letter late 2022 saying I did indeed have 3 years on my NI record and offering me to fill the gaps in my contribution record to enhance any potential pension. When I pursued it to the next level to maximise any pension, I got a letter saying I was ineligible to pay further into this as I did not have the required 3 yrs of contributions paid. I spoke with them by phone and was told that I did have ‘some’ record of contributions but their record did not match with my experience. I worked there for various different employers yet my NI record seems almost non existent and there are some ‘ghost’ contributions for when I was not in the UK some years after I had returned. When I enter my record on the Gov.UK website it tells me I have 3 years of full contributions. It then lists all years back to 1986, which incidentally was the year I started working there, saying that each year is not full and when I open each year it says “You did not make any contributions this year”. How can I try to get this rectified?

Hi johnny murray,
Without looking at your personal records I am unable to confirm if our records show the correct information.
If you think that the information on our records differs to the work you actually carried out in the UK, you would need to write to us and provide evidence of your employments and earnings/National Insurance contribution deductions.
The address to write to is:
International Caseworker EU Department
PT Operations North East England
HM Revenue & Customs
BX9 1AN
Please include the following information in your letter:
•    all employer details, which should include employers name and address, staff/payroll number (if known) and the tax years involved
•    evidence of your earnings during the period for example pay slips, P45 or P60 certificates
•    your personal details - full name (plus any previous names), date of birth, address, a contact number and your National Insurance number
•    your signature
Thank you.