Originally Posted by baroudeur

Quick search reveals...................................... you can:

• fill in form CA5403 and send it to the address on the form
• contact the National Insurance numbers helpline and answer some questions (you’ll need to fill in form CA5403 if you cannot answer the questions)

HMRC will post it to you and it will arrive within 15 working days.

Originally Posted by BritInParis

Give HMRC a ring.

https://www.gov.uk/lost-national-insurance-number

Originally Posted by EU.flag

... Its irrelevant how many months or weeks you worked in tax year, what counts that in that tax year your earnings exceed "Lower level of qualifying earnings", which is linked to
NI LEL. If earnings exceed NI LEL (even by just £1), you get one qualifying pension year. Figures change every tax year, for next tax year that will be £6396.