There is a lot of woodland around this specific area, and a free-range chicken farm just down the road on the other side. The field is on the left of the A229 if you are traveling south, Grandshore Lane borders the bottom edge of the field, but I saw the cat at the top end of the field, just on the edge of the crops and about 12 feet out from the hedge/woodland. It was about the size of a labrador, and was moving slow, quite low to the ground (a lazy loping movement) towards the road. Smooth coated, with a long thick rope-like tail and square-ish head. Definitely a cat, not a dog. I have domestic cats myself, even quite a "big" cat as domestics go, but this was definitely not a domestic cat.
Sorry I cannot provide any proof - or corroboration - but there really was nowhere safe to stop and even so it may well have slunk into the woods and disappeared by the time we doubled back.
Will keep a better look out in future!'