Engine cutting out

Hi, our Ford Smax was in the Ford garage last week to get the air conditioning fixed but whilest there it apparently got another fault; the engine cut out a few seconds after turning on the ignition. We had issues with it starting a few weeks ago and an independent garage replaced the starter motor. This immediately solved the problem and it was starting (and running) absolutely fine when we dropped it off last week. Now we are faced with a £350 bill for the work requried to fix it. This is a big shock as it was only in for the aircon (£60) and as we recently had work done and it was running fine we did not expect this! Any advice gratefully received - thank you.