I have a flat that I let to three men. One of them wanted to move out a couple of months ago so I agreed with him that he could find a replacement who would take his room until the agreement runs out in February. A new tenant was found, I gave him a replacement tenancy agreement - identical to the original but with his name on instead of the old tenant - but he said that the other two wouldn't resign it. I asked a couple of times what was going on, didn't get reply, summer and work got in the way and, stupidly, I let it slide. The new tenant has now decided that he wants to move out - commute longer than he expected 
- and, as he never signed the tenancy agreement, I wondered if I had any grounds to prevent him? As he verbally agreed to the terms, could I hold that over him. I don't want a constant churn of tenants.