They might have fitted them then, but unless they've only been on less than, say, three months, they could well argue that something came loose or has now broken and needs fixing etc. Probably won't if they fitted them in the first place, but they could.
In all honesty, you'd be better off getting a code reader off eBay (or even the RAC one out of the Argos catalogue) and reading the code yourself, letting someone on here diagnose it for you and then sort the problem yourself. At least you'd then start having an idea about your car and how to fix simple things without incurring the expense of a garage, if at all possible. We all gotta start somewhere matey, no harm in giving it a try yourself and then asking for help at all