In reply to gingerkate:
This bit of that page brings back memories for me:
"It is not normally possible to be a trespasser whilst legitimately on a right of way. However, if the user is not using the right of way as a route to get from one place to another, but using it for some other reason, such as to interfere with the landowner, they can be considered to be a trespasser"
I remember studying at great length in Land Law the case of a bookie who several decades ago was using a public footpath alongside some "rides" around Newbury (or it may have been Newmarket) to spot the form of various owners' horses. He was held to be trespassing, even though on a right of way, as he was using the right of way for a purpose other than getting from A to B.