Fiesta TDCI 1.4 will not run/start? 2002Reg?

To be precise ...it can start, but if I do not rev it, then it stops .....and it will not start again. It turns over fine, but will not fire at all. The pre-heat light does not come on (but as it is warm climate I presume it does not need to). It is intermittent, as most days you get in and drive off. When it does go wrong, ignition reset does not change anything, nor locking/unlocking car, nor even a battery reset. However, come back to it after a reasonable time and it just starts (but generally I'm hitting the accelerator as soon as it fires when trying again!!).