The tappets will contribute to it not starting easily but they would have to be a long way out to make it very hard to start. Are you sure the glowplug wiring is OK? I have in the past fitted new glow plugs, found no improvement and then found that the wiring was knackered, can't remember if the TD ones are wired in series or parallel, but if it's series if one bit of wire has had it you loose all the glow plugs after it.
Has it started suddenly or has it got gradually worse? If it was sudden, what's changed? Has it got good compression(make sure you use a diesel compression tester)?