If you have a circuit tester 12v what we do is remove the rail feed from glow plugs and connect earth connection to + terminal and tip circuit tester onto each plug to see if it lights,any that dont light up is a dead glow plug so would replace all 4.
Do you have a glow plug light which illuminates when you switch the ignition on? Is this light behaving normally, ie on for a few seconds, then off? If it is, then it is possible that the glow plug control unit/relay/timer has gone faulty. There should be 12 volts across the glow plugs when the light is on. I would start by checking this, first. No volts would indicate a faulty relay/fuse. Volts for too short a time would indicate timer/control unit.
If you have changed the glowplugs and it improved for a while, then it may be the timer unit. (new plugs, better glow for a short time, enough for an easier start.)
Have a look at the glow plug relay, relay 4a. You can measure the volt drop across the contacts with the engine running to give an idea of its condition, you could also clean the relay contacts (inside the can) with electrical contact cleaner, etc, or just replace the relay.
