I'm currently private renting and my landlord recently served us with a section 21 but yesterday we had a chat and he said only reason he wants us to go is so that he can sell he's lease, he suggested if we would like to buy the lease from him paying a certain amount each month until all is paid of I just want to know is this something we can do?

Do you not want to buy ? What do you want to do?

And where is this? Laws vary by area. If you landlord is the lease holder, he is already subletting to you. So I'm not sure what buying his lease does for you.

And where is this? Laws vary by area. ....

UK: Assured shorthold tenancy - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Section_21_notice)

I see nothing in this (http://www.communities.gov.uk/documents/housing/pdf/138289.pdf) guide which suggests that the landlord may not sell his interest, so I am puzzled at OP's statement that "only reason [LL] wants us to go is so that he can sell [his] lease". Obviously, if a buyer wanted to live in the premises, the OP would have to go first. But the LL could sell the leasehold to a buyer who would become OP's new landlord.