Intermittent Starting Problem Fiesta Tdci

My wife's car seems to have an intermittent starting fault. It's a 2007 1.4 TDCI model with approx 90000 miles FSH and due it's next service in a couple of weeks. On a number of occasions she has driven the car to town only to find when she returns to the car and try's to start it again it fails. On one occasion she called me to it and I couldn't start it either was turning over fine but not firing. I left it a while and phoned to get it recovered I then tried it one more time whilst waiting for RAC after about 20 mins and it then started. No warning lights flash up and temp whilst driving is normal. No issues when driving and car is used daily with fault happening about 8 times now over the 4-5 months. The fault has only happened shortly after car is driven she states that it always starts again if she then leaves it 20-30 mins . We have taken it to a local garage but they said as the car starts fine with no fault codes then they can't fix it until it develops the fault again. The car will be going for a service end of this month I am hoping someone may have come across a similar fault before so I can pass info on to garage when it goes in so we can get it sorted. Any info appreciated.