I think your all a little niave.
As long as the tenant has the keys to a property worth a couple if not a few hundred thousand - he or she is calling the shots.
Threshold / PRTB / God / your mother cant help you. Diplomacy and sharp practice are your only answer.
1. Call up with two friends - dont tell the tenants - and just walk in. Should the tenants say anything confrontational (you will know that they are taking the Pxxx. Just apologise and ask if they received your message notifying them that you would be viewing the house with prospective tenants. Ignore them and take your new tenants around the house.
2. Tell the old tenants that the new tenants will be moving in on Friday. Dont take no for an answer. Follow this up by pointing to a few stains here and there. Ask will they be getting those stains cleaned before Friday.
Follow along that route, and keep smiling!
If you still get no joy send them a copy of a civil bill (marked in small typeface " without prejudice"). I usually find that this scares them into going. You can also send them (the following day by registered post an application to the small claims court for damages. This is not worth the paper its written on - but chances are they dont know that.
Next time you are renting a property, dont tell them your the landlord. This means that should problems arise you can always say "I'll have to speak to the Landlord - but I dont think he will go along with that". This will give you time to think.