suspected cannabis factory

Possession - Section 8 and Section 21

not too sure what to do here.
i have a tenancy where i couldnt get access to do annual gas cert because the tenant had changed the locks. Rang the tenants...They said they were on holiday and back in 2 weeks... fine. Gain access with the gas man. Smell of cannabis every where. Windows open. House doesnt look lived in. Still the semi obvious signs of factory behaviour xmas tree in front lounge etc. No signs of actual factory in progress. I suspect the 2 weeks gave them the opportunity to empty all the grow rooms, tents etc.
I want to end the tenancy. On what grounds should i use? s8 or 21? how about both at the same time? Can i use s8 as a minimum because they wont give me a copy of the new keys etc?
please note i havent actually contacted them to say i suspect cannabis factory etc i didnt check to see if they have bypassed the meter

thanks in advance