Teenagers discover a network of tunnels to other times

I’m trying to identify a novel for young adults about travelling through time using a maze of time tunnels. I borrowed it from the Town Library in the early 1970’s, so it may have been written during the late 50’s or early 60’s.

I think it’s set in the North of England, possibly Newcastle. Two teenage lads fall through undergrowth into a large chamber. They can’t climb out, and the only way is to walk through a tunnel at one end. It becomes a maze of tunnels with glowing walls, and when they reach the opening at the far end, they have travelled back to pre-history.

They walk back through the maze, and then through again, and come out at a different time. They repeat this exercise, and find that taking a different combination of left and right turns takes them to a different destination time.

I believe that, at one point, they were at 5,000 BC, and rescue a girl from being sacrificed.

Their final journey through is to take a lot of right hand turnings, so that they end up a long way in the future. The exit of the tunnel is manned, and the attendant tells them that "The last person to come through was a Roman soldier". They are then anathematized, and sent back to their own time.

Two years on, and there has been a power station built over the chamber, with the chamber itself housing the reactor.

If anyone can identify this book, I would be very grateful. I can then also buy a copy through Abebooks!