Question Author
Having discussed matters with legal experts the opinion is as follows.
The tenants are in breach of the tenancy terms and conditions that were signed on taking occupancy. Specific clauses were highlighted and initialled by the tenants as having been read and understood, one of which concerned the issue of smoking within the property and the consequences of doing so. In short, there was, and still is, no ambiguity. Therefore, the experts believe there is a case to pursue, since the tenants may have falsley created a belief from the outset that they were non-smokers to secure tenancy, or that they have subsequently decided, by intention or neglect, to ignore their tenancy terms. However, from a criminal point of view, the experts are less precise. The likelihood of proving intent is not impossible, though still difficult, using the specific clauses as evidence. The likelihood of proving recklessness is somewhat easier given the same evidence. None of the experts wants to be brave enough to pursue a criminal case (possibly because it's in the too difficult tray, I suspect), but all have said it would be interesting to see how it would run through the system, given the current climate surrounding no-smoking enforcement in other areas; albeit this is not a public building.
So, their overall opinion is to pursue a breach of tenancy through the civil route because it's less ambiguous;
'Were they allowed to smoke inside the property'
'No!'
'Were they aware it was conditional'
'Yes, they signed to that effect'.
The matter is far from resolved because they can't decide how to make progress from our discussions. One is pondering over a fag and a cup of cold coffee, the others are debating whether it's a liquid lunch, or something from the local greasy spoon. These are experts, mind...!