I am trying to find a sci=fi/fantasy book from a vague plot description.?

[extramural]

The plot concerned a group of people who went on a day trip to an old ampitheatre. Once there they meet up with a coach party and they are all seated in the theatre which is open air and it moves. When they get over their shock they realise that they have travelled a few moments back in time. The ampitheatre keeps moving and they keep travelling further back in time. Each time the theatre comes to a standstill it has shifted in space as well as time and thus there are several casualties/amputees who have the misfortune to get caught in the moving theatre mid cycle. The book culminates in a battle at the end of the book between those seasoned time travellers who have been moving for many years and the new arrivals. I have no idea what this was called, who the author is or when it was written but i think it may have been set in the UK. Can anyone help?