Why doesn't my ford mondeo start when the engine is warm?

I have a Ford Mondeo TDCI 55 plate, it drives fine, but when the engine is warm (driven over 2 miles), I have great trouble starting it again. Starting it in the morning, or when it has been left for a few hours, is fine too.
I only have trouble when I have 'popped to the shops', and the engine has been off for 5 minutes. It's driving me insane, can someone please help.