What is Britain's Post Office IT scandal about and who is involved?

Prosecution of thousands of people has been called one of the greatest miscarriages of justice in British history

Toby Jones, centre, stars as Alan Bates in ITV's series dramatising the Horizon IT scandal Mr Bates Vs the Post Office. Picture: ITV

It is a scandal that has rumbled for years, and yet it is suddenly dominating British politics. Here is a guide to the saga of the Post Office and its Horizon IT system.

What is it all about?