had exactly this on my 53 plate td4. got harder and harder to start through the day, next day, just gave it bollocks all together and refused to start. My local independent garage could get it to fire and run everytime using easy start (not desirable, but they had to move it about and do diagnostics). Turned out it was a failed injector causing excessive leakback. While they had it, had all 4 taken out and refurbished, as I didn't want more down time for the same fault. Since then touch wood....
Good luck to you