Many thanks for your help. Yes, they have charged me for the additional hours.
Forgive me if I am misunderstanding your question. The job did take them nine hours, however, my flat-mate who witnessed them working assures me that they could easily have done the work in half a day if they had hurried up. I believe that it was a four hour job with the right sized van and two efficient and trained workers.
Thank you so much Jo. This is such a help.
Just one clarification.
Because they have already taken the money for the full nine hours out of my account, if they refuse to repay any of it - although they have agreed over email to reduce the cost by one hour - do you think I'd have a chance of getting it back if I threaten to sue?
Thank you so much Jo!