Stalls while driving. (And when stationary)

I was wondering if anyone can help me! I have the vehicle listed above Tempest model. It is driving me crazy! It keeps cutting out whilst I am driving, all the dials go down and electrics go off but sounds like the engine is still on. When I turn the key to start it back up it makes a terrible grinding noise (like the one when you turn your engine on forget and do it again). It also does not start every time when it is stationary. I am too scared to take it on any long journeys encase it does it on the motorway. Unfortunately when the mechanic has had it it would not fault! We have tried everything we can think of to make it fault ie; driving faster over speed bumps, using all the electrics at once, driving in to low/high a gear and it would not do it. Checked the battery connections (which is new), fuel injectors and the sparks and everything looks fine. There are no warning lights.
Sorry for the long post. Can anyone help? Thank you!