This is a Carboniferous plant fossil! I'm not very good with Carboniferous plants but it looks to me like the common genus Neuropteris sp. This is a common preservation where a nodule formed around the leaf, which has now nicely split to reveal the fossil. It looks like it was on the beach for a while before you found it as it is a little weathered. The Carboniferous period was long before the dinosaurs. This was from the late Carboniferous coal measures of around 300 million years ago and where you found it is right on the Durham-Northumberland coalfield.
Congratulations on your find!