All this legal nonsense is just bluster (and yes that is my professional opinion!). Don't think on it any further.
I would ask that the letting agency kindly do not continue to inform you each and every time one of these tenants raises a whimper. Any issues the tenants have should be put to the landlord only and are not you concern. You do not need the stress of being updated continuously.
A short, factual letter addressed to each tenant setting out the legal position and reminding them that their issue is with their original tenancy agreement and in turn their landlord would be a good way of ending things. Keep it brief, to the point and don't invite correspondence on the matter.
This really is the landlord's issue and I suspect he in turn will have a claim against either his solicitor who drew up the tenancy agreement or even perhaps the letting agency if they have used a "one size fits all" tenancy agreement of their own.
However, none of that is your problem OP. Tell the letting agency to deal with it, send a letter if you think it appropriate and then disengage.
I do agree that any further contact with you by the tenants should be logged with 101, aggressive or otherwise.
You also have grounds to contact the landlord yourself if such behaviour continues as the tenants will be breaching their tenancy agreements with such appalling behaviour towards their neighbours. But that is a whole other angle that could be used as a last resort.
Finally - I hope you're putting your feet up!