Fire at property

Possession - Section 8 and Section 21

Last week my tenant returned home, drunk, in the early hours, put a pan on and fell asleep. Thankfully the fire brigade, got him and his partner out, ok, apart from smoke inhulation. The property is virtually distroyed, smoke damage throughout and kitchen gutted. The couple are staying with familly. The loss adjuster estimates 16 weeks at least for insurer to make it habitable. The couple have handed keys back, as we don't know how safe structure is, and for safety I don't want them visiting. I do not want the couple back, as he drinks alot, and I fear it could happen again. Whilst they are staying with familly, and given the fixed term AST has passed, am I legally ok to issue a section 21 now, so that by the time property is put good, the noticed will have well of expired, and if they then wanted keys back would I have to give them back, and get a possession order.

Thanks