Towcester, Near Silverstone. https://jackshillcafe.co.uk/ < Scroll down. I believe it's first Friday of every month... I will probably pop over this Friday if it's not raining. As it gets darker and colder I guess it will perhaps get less busy. I've only been once before and it was packed, apparently in summer they sometimes have live bands? It has a bar and other norm facilities.
Thanks for helping me join up the dots! - have been a few times to 'The Super Sausage' Cafe and bikers have said they were popping down the road to another cafe further up the A5. Must give it a visit next time.