As far as I know, you should not be sharing your NI number with anyone other than the Inland Revenue or the NI contributions office.(posted 7844 days ago)Lenders might have needed NI numbers in the days of MIRAS but those days are long gone.
I'd suggest that you don't supply it. Well worth ringing your district tax office to see what they say about the need to keep your NI number confidential. If you get an answer, please share it with this board.
Lee