I think your case is good Pete.
the agreemant included the property was to be empty. A verbal agreement, always difficult, but never the less, an agreement.
It was'nt empty, your tried within reasonableness to get the persons attention.
You tried your best to contact the others.
Best endeavor.
Just try to reason it out with them, difficult don't I know, but then verbally and confirm in writing what you are going to do next. ie give them a date you expect payment and then small claims court.
Your £45 fee for going back is within reason too, IMO.