C3 diesel cranking but not starting

[Sparlings]

So starter was replaced a month or so ago as completely dead. Fine for a month.

A week ago car was failing so start in mornings - cranked fine but just wouldn't ignite and start. However, trying few hours later and cranking several times for a little bit longer than normal and it started. I deliver in the evenings so multiple stops and starts with no problem at all. This went on a few days. I was thinking maybe too cold outside first thing and glow plugs circuit maybe not working although no EML.

Then stopped starting in afternoon even with outside temperature above 10 degrees. Cranking multiple times and nothing. Seems to be cranking at normal speed unlike if weak battery. Checked battery charge and around 12.5v.

Any clues as to what might be wrong? I do have an oil leak from somewhere showing under engine so perhaps related?

It's an old 2010 C3 1.4hdi with 160,000 miles. Anything major wrong and will sell for scrap.

[PaulC5]

Maybe a fuel problem or something like a crank shaft sensor fault.

[moizeau]

If the oil leak is near the crankshaft pulley a quick and cheap job would be to unplug it and give both sides of the connector a good spray with electrical contact cleaner. If this doesn't work you'll need to get it plugged in before throwing parts blindly at it.

[Muir]

A diesel with decent compression should definately be able to start at 10 degrees even without glowplugs, so your issue is quite likely fuel related like PaulC5 hinted.
Have you smelled the leaking fluid to determine if it's indeed diesel?
Also try to trace the fuel lines all the way from the tank to the injectors themselves to see if you can spot any leaks - pipes usually run the length of the car either in drivers or passengers side.
Especially leaks on the high pressure side will disturb the engine badly as very low volume are pumped to the injectors.
HP pump needs to build high pressure to pump fuel through the nozzle tips vs. the cylinder compression.