keep a note of everything, dates, times, conversations, including the fitters information and his side of the story.
send a letter straight away to the client outlining all you have done and put on it the cost to you, ie the fitters rates, replacement glass etc and how basically the customer failed to assist you, ie not being at the agreed time
If the client tries to claim more refuse to pay
I believe under law you have a right to rectify the problem before a client can just decide to do it there way, ie more expensive way so all this information will be needed to justify not paying the difference, so if your bill is £200 and the clients £600 then you may need to contest it