A faulty alternator is not going to prevent the engine from starting.Either the batteries you have are not adequate for the job on a diesel or there are other possible issues with the starter motor or wiring connections.

Arrrrrgh!!!! bought a new alternator £90 out the thing in and........................NO difference :( that bleedin eng man light still comes on like it's trying to stop me from further damage ??????? a friend of mine had the same problem on a peugot diesel and he changed the Starter motor and it cured it......... i really really hope this isnt the case as the s/motor looks like the b@st@rd of all jobs.........i'm gonna ask a mechanic i know to bring her diag stuff home and see what it finds............ I did actually get a friend to check the battery and alt and they seemed to be ok????? i gave it a go today and it turned and turned but no joy....i connected a spare battery up to the one in the car using jump leads and the car started????????????? starting(pun) to drive(pun) me mad now......................Anyone got any other ideas apart from scrap the thing???...cheers.

It could be the starter motor. The ECU will need to see a certain fuel pressure in the common rail before it will open the injectors.

If the sterter is slow it may not reach that critical fuel pressure.

But guessing can be expensive & the only way forward is to get the codes read & go from there.

A larger capacity battery will be fine.