How to repair a striped glow plug?

My renault megane has stripe its threads preventing me from replacing the glow plugs. I have seen repair kits for this but they are just so many different sizes, just need to know what is the best kit to buy and what size. I was told that these are a better solution to having it coiled. Plus i would like to repair it with out removing the cylinder head.

jps
January 2010