Spouse Visa Application in 2017(overstayed from 2005-2008)

[soulking]

Hi All. This is my first post on this forum and its been very helpful in getting my head round this spouse visa application process.
I am a Zambian national and got married to my British wife last year (2016) in May. Before we got married we maintained a long distance relationship for 3 yrs and she visited me 3 times during that time. My question is in regards to my immigration history.

I entered the UK in 1998 on a Student Visa for University, I graduated in 2003 with a Bachelors Honours degree I then undertook another course from 2003-2005 and renewed my visa accordingly. But unfortunately I then overstayed in which I worked illegally for 3 years and left the Uk in 2008 voluntary at my own expense. Nothing was stamped on my passport as well.

During my legal stay in 2000 I was issued with a National Insurance Number to undertake part time work during university holidays etc.
On my application I am going to declare that I overstayed my visa. But there are questions with regards to my NI number which I have forgotten (it being about 9yrs since I left the uk). Which is the best way to answers cos if I say yes they want the number.
Will my overstaying, working illegally and my not knowing my National Insurance Number impact my spouse visa application.

Thank you in Advance for you help