Matthew,
A footpath is still owned by the landowner and by agreement he allows people to pass and re-pass so if that's all they do, it's legal.
If they loiter and disrupt a lawful activity then yes it is trespass, in fact aggravated trespass which is a police matter not a civil one,
If their only intention is to film you to goad you into some form of reaction or disrupt your lawful past times, then that easily comes under harassment, however it needs to be a "course of conduct" which means it must happen more than once, again that's a police matter,
There is an interesting case