We rent our old house out. The owner of the neighbouring property is accusing the tennant (who has now moved on) of dumping internal doors and a mattress over the back wall and into his garden. The letting agents spoke to the tennant who unsurprisingly denied it, saying that the garden has always been filled with rubbish. The alleged dumping ground is owned by this neighbour who, for reasons best known to himself has allowed the property to become derelict, complete overgrown and stands empty. He actually lives in the other house that neighbours the empty one. He has informed the street enforcement team, without evidence, that this tennant has dumped these items and they have written a strongly worded letter to US as the owner of the let property, saying we need to get rid. But there is no proof that this stuff came from our property. Where do we stand? I'm afraid there is no goodwill between us as his derelict property has caused subsidence in our property, so in no way do I feel like sorting it out for the sake of good neighbour relationships!
Maybe respond denying the accusations but explaining that you've pointed your CCTV cameras at the wall now and will keep an eye on any futher activity.