Catherine, We have had a recent similar example We received assistance from our Council Tax and Housing Benefits offices to find what details were registered for the relevant 'permanent' property and any claims that may have been made. Finding out where people supposedly 'on-holiday' were working and the geographic relationship between the 'holiday chalet' and the more distant 'permanent address' was helpful. We also received assistance from another authority who checked the state of a 'permanent' residential address (that was in their District) for us to see whether it appeared to be occupied, and if so by whom. With such evidence we have been able to have a 'useful' conversation with the occupiers of the holiday home and the owners of the site. I hope this gives you some useful ideas, I'm sure there must be plenty more tools you can find too.
