EC - previous time in UK & NI number

[hsmpkiwi2]

I am looking at getting an HSMP in a couple of months.

I have previously lived in the UK on a working holiday visa. I did not obtain a NI number, however I did pay tax for 6 months during the time that I was there.

Is this going to have any bearing on me obtaining EC, ie: will they request my NI number given that I have already been in the UK on a WHV?

How closely will they scrutinise my past time spent in the UK, working and otherwise?

Is it essential to provide previous income tax returns when applying and how far back do they need to go?

Thanks for any advice.