From experience of similar situation a few years back with a very well known chain.
Get a paper trail for everything. Record dates, prices for parts, details of what happened, jot down conversations with the company on the phone, in person and who you spoke to.
Once you have a detailed breakdown of prices and labour, speak to another independent mechanic. Explain what happened. If he/she thinks the work is unreasonable and over priced or that they are at fault, make a detailed complaint to the company.
If that does not work, you could contact either Consumer rights, the ombudsman or there is an association that officiates all car dealers (apologies but cant remember what its called)