UK Landlord Forum / Life

AST was in place but just runs on now. Rent never increased in 5 years but grossly under marketed now (was through a council scheme), by £200pcm 
Tenant now moved in partner and another child also.
How do i go about increasing the rent to a market rent (even if i stagger the increases). and should i still go to two months notice of increase etc.
Tenant came off of benefit  so now between him and me  as opposed to Council third party.

Advice gratefully received please !