Best thing to do phone around for a price.
First look underneath your car and see how many sections it is in.
(Important, check the condition of the rest of the system!)
Ie, do you have a rear silencer and centre pipe combined, or just a rear silencer.
Then phone a motor factor, tell them the, make mod year if they are computerised they will need the reg no to look it up.
You will now have the trade price, so when phoning round the fitters you know how much mark up they are trying to add.
This place is in your area, but phone around first
http://www.georgetyres.com/