Focus starting issues

I own a Mk1 2004 Ford Focus 1.8 TDCI and ive owned this for around a year. Its cover 84,000 miles.

In the winter, last year, it would struggle to start on cold mornings, but would always start and then be fine starting thereafter. Summer came, no issues.

This recent cold spell has brought out the same symptoms, but also it did lose power while driving down the motorway, with the amber coil light flashing. I stopped and restarted with no issues after that. I decided to get it sorted and fully expected them to tell me it was the glow plug needed changing.

They didnt and told me it could be an intermittent sticking injector, as for the time it was in the garage it would start and run fine. I suspect it was left inside the garage for the few days it was in, sheltered from the cold. They suggested adding some fuel cleaner and running it for a week or so and repeating.

The next day it struggled to start as per, so i booked it in to have the glow plugs changed. All went well after the repair and the car fired up with no issues.

Yesterday morning, the first use after the glow plug change, the car started after a few attempts, but cuts out straight away after starting. Wont even let me put some revs on, just dies??

As youve probably gathered, cars arent my thing and havent a clue where to start .

Any advice will be gratefully appreicated .

Michael