As an update:
Noticed a smell of smoke from his room - badly covered by incense, ambi-pur and deoderant.
Went in when he wasn't home there a bong in his room, just plain smoking is a violation of our lease which could get us kicked out. And legally the responsibility for it is on our heads, eg police have a look around find pot then as the lease-holders we would be charged with possesion. Something that I doubt would look to good on our records and would hamper job prospects.
Oh and we also found out that he'd been leaving his heater on all day eg when he was out. The room was so hot that I checked if he had a hydo lab running in the wardrobe before I realised it was the heater. The issue with the heating being that he would be using say half the power but only paying for a third, and I have no particulat desire to be scimping on heating for myself so that when the bill comes we can afford for him to be toasty warm 24/7.
We gave him two weeks notice based on the pot, and will probably keep most of his bond. To pay for his part of bills we'll get after he's gone, cleaning bill and a weeks rent when the room will have to aired to get rid of the smells. Incidently the smells are so bad as to give my other flatmate nosebleeds.