Renault megane 1.5dci starting and cutting out troubles?

Hi there I've had a megane 1.5dci for the last couple of years and has been brilliant. The last couple of days it's been really hard to start and was low on oil, I've topped up the oil( made sure it was correct for the car) but still struggles to start. Then as i was driving it cut out. I've taken it to a garage who plugged it in and said there was a fault with the glow plugs. So they've changed the glow plugs and still no luck. There was also an immobiliser fault and they said it would be that. But they also said only a main dealer would be able to fix it. But I've read online the Renault immobiliser systems won't make the car cut out, they only prevent the car from starting.

Can someone help?
Thanks