Seat Ibiza will not start sometimes

I would start with the basics and move along from there. IF you can find what it isnt it may make it easier. Have you checked the radiator/bottle and pipes for leaks/blocks? Then i would check the thermostate which controls the waterflow through the engine amoungst other bits. The engine does have a cut out in case of overheating so to start i wouldnt worry too much at this stage about the not always re-starting when hot. Although checking the plugs wouldnt hurt. If all that checks out-try listening for or checking the fan in kicking in. Or maybe a mixture of any of them. An over worked fan but a stat that isnt opening......