Hot day

Hi has anyone come across a problem with MK2 1.4 Petrol Octavia, not starting on a hot day. It could take a good 5 mins to start. Once it starts its fine for the rest of the day. It only happens on the first start from around 25 degrees. I am having trouble trying to get it sorted in the garage. they put it on the computer nothing shows up. When I suggest they say no never heard of it but won't do it. I was told by one guy that the thermostat sensor needs to be changed, because the ECU is getting a false reading.