Escort 1.8 Diesel Starting probs?

My daughter has an old 1.8 diesel.
Doesn't start in the morning without lots of cranking even though the glow plug light comes on and goes off before starting. I know the glow plugs haven't been replaced for years could these be the problem?
The car started this morning after I fully charged the battery.As in previous threads I'm thinking check the glow plugs and replace these before heading for the battery. I don't think fuel is a problem as when it satrts it does run fairly smoothly.
Any advise welcome.
Regards
RT