Fences...

Was planning to put up a new fence. I assumed that the fence, to the right of my property, was my responsibility but my neighbour who, years ago said it was is now saying it's her fence and we shouldn't have attached things to it [my ex built a cat pen and a lot of it was attacked to the fence posts]. From memory, we were the ones that put up that fence when we had a dog [need to speak to my ex about that]. I've always got on well with my neighbour of 40 years but this has really upset me as she now wants another fence panel replacing which will mean me having to dismantle parts of my garden . Is there a hard and fast rule about fence responsibility? From googling it doesn't seem that there is. My house deeds went to the solicitor when I was divorced and they are now on the Land Registry computer and I will have to pay to get a new copy. What was meant to be a simple job is now turning into a nightmare...