A word from the wise....
Disconnect the battery completely before changing the glow plugs
When buying the plugs make sure that they are matching make, brand and numbers.
Re-attach the battery, attach the ignition rail to all the new glow plugs and place them on a baking tray to prevent any heat damage. Turn the ignition to MAR.
All you want to do is to test them quickly to see if the glow/heat process is uniformal so that they all glow/pre heat at the same time frame and of the same illumination (heat consistency)... It should take a maximum of thirty seconds to test.... Once you are happy, turn ignition off and disconnect the battery and reinstall them.
It is not unknown to have one or even two glow plugs in a batch to fail even when new, this test will show any weaknesses before fitting and then throwing in a red herring before going hunting for another fault when all along it was a duff new glow plug.
If one is not working as it should, simply take it back to the supplier for a replacement, but unfortunately you will have to check it all again... Just in case
Been there, done that, got the T shirt mate
Hope this helps
Almost forgot.... It may be sound advice to also change the heater plug relay at the same time and clean the contacts up... For the cost of the relay itself, you then have peace of mind the the circuit is in good working order for at least three further years.
PS... stay away from using Ether (aka easy start) Diesels love the stuff but once you use it more than once, it will always need it on cold mornings like a junky needs a fix