Sorry c6em, I posted before seeing yours.
I only had one potential REIN comming from an address where they wouldn't answer the door. As far as I understood the section dealing with REIN faults in Leeds would pass ourt findings to offcom who are ment to follow this up. They have their own interference guys with the powers to "lean" on any parties found having to broadcast REIN. Be it affecting telecoms or anything else.
Generaly I've found 3rd parties always approachable, curious, and helpfull. Also as typicaly it would affect their own service as well and feel grateful that they now have a better net connection.
Although it was outside my remit to approach 3rd parties it was bar one exsample successfull. Just a case of being curtious, explaining the situation while avoiding any remarks which may be taken as accusatory. Also its good to be discrete. I can think of a village which was knocked out. It was hard to hide the fact as I was being watched by many villagers which house had the offending item causing the problem (this case a netgear router, probably damaged due to a lightning surge). Just reassure everyone that it can happen to any appliance in any building. things breakdown but REIN is an invisible breakdown to us.