Hi Prof Eman,
I am not sure of the sum in question do you mind disclosing it? However, my advice would be, unless it is a lot of money (say £1000+) forget it and move on!
It never ceases to a)amaze us what a mess other peoples lives are, b) what a mess they can make one's own life, and c) that any type of litigation or action against tenants is loaded in their favour....they generally have nothing so they have nothing to lose, they get legal aid you spend hours trying to litigate in person to mitigate costs or even if a CCJ is brought against them they don't pay and if the person has a criminal record well, what more can be said!
We have walked away from as much as £10k worth of damages and unpaid rent because it got too stressful. We are in the lucky position now to interview all our tenants, and don't take any we don't want, we only accept a minimum of two months deposit on the basis if they can't find it now how could they if they became unable to pay the rent if they hit a bump, and one months deposit doesn't go that far in respect of damage or neglect.
There argument is an interesting one and it (again) never ceases to amaze one what get thrown up. However, my feeling is that unless the property was, by reason of any action, or inaction, by yourself then the rent is due and payable, there is an AST in force and it is not an hotel.
In these circumstances a call or email to the charity Shelter might be worthwhile as they are very tenant biased but could well have a view on this.