How did you test the glowplug? A good plug will have a resistance of about 0.5 ohm. If the plug is OK, then maybe the connector or wiring may be the culprit, or possibly the glowplug module. However, a single glowplug failure wouldn't contribute anything to your other problems.
Thanks. I checked for zero voltage drop through the plug while in situ but have since realised that it isn't a particularly reliable test. The only way I thought it could be related is if there was an issue with wiring. To be honest, it isn't my main concern at the moment.