Mercwarrantyman. I think you're trying to hard to find a fault. This may or may not work, but both my suggestions are cheap to do and you can do them yourself. Firstly I have glx tdi for about ten years with the mileage around the 350,000 and have come across most things mention on this website, so here goes. If the engine runs and then just stops, say during a journey try replacing relay 109 in the fuse box, you may be able to repair it by opening up the relay and re soldering the connection that has freed itself, hence the situation you leave the car for a while and it works fine (relay cooled down and re-soldered its own connection!) Second area to look at, this one drove me nuts but got there in the end. Symptoms, engine would start then stop after 2 or 3 seconds, drivers door alarm light would flash quickly for about aminute, then go out. Disconnect the battery, remove the plastic covers around the steering console, pull out the plug which lead from the key column ( left handside of steering column as you sit in car) check plug for dirt or damp replace making sure of good connection, I put a plastic tie around it to stop it moving again and insure good connection, replace battery, start engine, replace plastic console, go down pub and rejoyce, it's that easy. Hope it helps.