As said diaphragm holes in the filter head are common, allowing air in.
Have you checked for POWER at the glow plugs as may have a electrical problem(broken wire), have you drained the water off lately from the filter head ?, try some MILLERS diesel cleaner awsomely good stuff, as is BIO DIESEL which has a massive cleaning action, so much so that they can only stick 5% bio in normal diesel.
tabetha