Done on a JTD 1.9 8v 115bhp.

Usual disclaimer... I'm not responsible if you still manage to snap a glowplug or cause any other damage to your vehicle whilst doing this!!!

I recently had to replace the glowplugs on my JTD, but two of them were seized. They were so bad that the diesel specialist I took it to were certain they would break if they tried to remove them.

I had no choice, so decided to take a look myself and see what I could do. First thing! make sure you give your car a good run and get it up to temperature... this is much easier on a hot engine!

Glowplug Access
The glowplugs are notoriously difficult to get to... you need to remove the plastic engine cover, the EGR valve and the fuel return manifold.
The EGR must be disconnected with the grey connector by the actuator. Then disconnect the exhaust gas supply pipe near the heater matrix pipes. It is a clip similar to a jubilee clip with a hex head bolt. Then remove the pipe in to the inlet manifold with 2 hex head bolt - remember to recover the gasket. 3 13mm bolts hold the EGR to the inlet manifold.
To remove the fuel return manifold, with the car left standing for a while to lower the fuel pressure, unclip the hose between the fuel filter and the manifold. Have a rag ready to soak up any diesel! Then undo the 2 10mm bolts holding it in place and swivel it up and out of the way - be careful though... the hose tails on the manifold break easily!

Glowplug Removal
You should now be able to see all 4 glowplugs... disconnect each one. Try removing them with a 10mm socket but be gentle... if they're not shifting, don't force them or they may snap!! I had to use a series of extension bars and a flexible joint to be able to use my ratchet on them.

For the ones that aren't budging, use PlusGas. PlusGas is a very effective penetrating oil that will help unstick the plugs. WD40 will probably do nothing at all! To get the PlusGas in to the wells that the glowplugs sit in, I bought some aquatic airline from a pet shop (99p) and warmed up the end in a cup of boiling water to allow it to slide on to the nozzle of the PlusGas. I then taped the airline to a length of thick single-core electrical cable to allow me to shape and hold the airline in to whatever shape I needed to get the end of the airline in to the glowplug wells. The PlusGas will flow better if you warm the can a little.

Fill the glowplug wells with PlusGas and leave over night. The next morning, top up the wells.
That evening, try removing the glowplugs... again BE GENTLE! If you get a little bit of movement, try turning as far as you reasonably can back and forth to work the PlusGas down the threads.
Mine hadn't improved much, so I reassembled the car and ran it hot again, then repeated the process. This time, I left the glowplugs in the middle of their movement range when I put the PlusGas on so that the threads were as open as possible to allow the PlusGas to get through easier.

It took me 3 runs of this process to get my glowplugs out, but they did eventually come out. And these were glowplugs that a diesel specialist were certain would end up snapping! :) PlusGas is amazing stuff!!!

Glowplug Replacement Choice

Sounds obvious, but I originally bought a set of NGKs since they are supposedly good quality plugs and I wanted to make sure I had a decent set in so that I wouldn't have to replace them again for a long while.

First thing I did before I even installed them was to test one on a battery (just out of curiosity to compare them against the old plugs.) They were relatively slow to heat up, and I mean slow enough to get me concerned enough to go out to buy an OEM plug to compare them against. The OEM plugs are Bosch.

Connected in parallel to the same battery (as they would be in the car,) the OEM plug heated up substantially faster than the NGK! It also appeared to reach a higher temperature (by glowing more) and after disconnection, retained the heat for much longer. As well as that, the shaft between the head and thread of the plug is thicker on the OEM Bosch ones, meaning they are far less likely to break when you come to replace them!