2008 Fiesta 1.4 TDCi style, intermittent fault, engine stop

Hi, This is an ongoing issue, electrical I'm pretty sure.

Fist time this issue showed. Car running normal, I park at a shop, come out, car won't even turn over, dead, five mins later, starts and runs fine, this was over a year ago. let it develop over time, now when your driving, randomly battery light comes on, engine dies, keep it rolling ingear, goes normal again after a few seconds.

Happened once coming home from mates, red light, engine dies, comes back to life, dies at traffic lights, tried starting many times while red traffic light was on, would not even fire, just as the traffic light turned green, it started and drove home 2 mile, no issue.

Then this weeks shopping trip, tried this thing again 3 times in 1.5 mile, kept it rolling in gear and it came back to life everytime, got shopping, it drove home like a goodun no issue. When I took it to a local garage in the summer, they did not know what the issue was, waste of time going there again. All very worrying for a 70yr year old who is now not sure when he will get home IF he goes anywhere. I 100% dare not go on a motorway anywhere, making me loose sleep worrying about it now. fingers crossed someone knows. PS I'm 100% sure it will start if I go out to it, nothing ever happens when it's stood outside house running, only when it's moving, which is weird, something loose I have not a clue, never come across this before with any diesel engine I've been driving, it's not fuel filter, had that changed.

cheers

Rob