Thanks for all the input. Good advice in there - what I'd like to do and what we will do being two different things. Getting a camera installed on the roof tomorrow to start gathering evidence. Also getting a neighbour to take some photos of rocks that keep being thrown over, damage to property etc. Trouble is, even if evicted, what's to stop them going back in the dead of night and setting her house on fire. These sort of people - especially when on drugs - know no bounds, and are bound to know far more 'heavies' than I do.
Also going to contact the H.Assoc in the morning to see what their take is. Maybe contact the police, but remember another local case where a mum and disabled daughter had been harassed for years and reported it many many times. In the end they parked up in a layby and set themselves on fire as they couldn't take it any more. The police were "really sorry" ......
I'll let you know how things work out, if indeed they do.