206 diesel won't start - turns over but doesn't fire

[pspinks]

I'm having problems with my 206 (diesel), which sometimes will not start. The engine turns over, but it doesn't fire. On the dashboard, the STOP light stays on, but checks on oil and brake fluid show everything is OK.

The first time this happened, I called the AA. They arrived an hour later and the car started first time (grrr!). The AA man explained that there was sometimes a problem with the immobilizer, and explained a trick of pressing the key fob button for a few seconds, which he said could clear the problem.

The next time this happened, I tried the trick. It didn't work, so I called the AA again. He arrived after an hour and the car started first time again (grrrr!). He also thought an immobilizer problem was likely, although it wasn't possible to be certain. I was visiting family 100 miles from home, so I made a note to book it into a garage when I returned home and get the immobilizer sorted out.

On returning home yesterday evening (i.e., before I had a chance to get it to the garage) it happened a third time. This time I just left it for an hour and it started first time for me (grrrrr!). We can survive without it for a few days, so I'd like to get opinions of what might be wrong, so I can brief the garage mechanic with the best theories.

I have two theories of my own. Theory A is that something heat-related is causing the problem, because in each case the car had been driven earlier, and had been standing for 5-10 minutes. Theory B is that the immobilizer is being confused by random presses of the key fob button, which wasn't in the ignition switch (I was using an ordinary key) but was in my wife's handbag on each occasion (and who knows what might have been pressing on it inside there).

All suggestions gratefully received.