Hi Mark,
First of all why do you want to change these glow plugs? Does your car find it difficult to start in the mornings? If it starts fine, you do not need to change them. If they need to be changed, my honest advice is to take it to a good mechanic who can help.
If you are from London I can direct you to a specialist who is honest and reasonable. Good luck.
Aziz, March 2009