There's a rough video here which shows the Toothill Long Barrow in Halliwell, Bolton. There's no doubt about its authenticity but it is suppressed, for various reasons. It's 80m. long, 25m. wide and a little over six high. It's part of an ancient farmed landscape and is unknown.
https://www.youtube.com/watch?v=MAGFpX9BsuM
Comments welcome - there is a What 3 Words link in the description.