Ml 320 Spark Plugs

[inveresk]

Trying to change the plugs on my 1999 ml320 and having some difficulty. I'm using a long bar plug spanner but am having to exert quite unusual force to free the plug and I don't want to break something. I wondered if somehow the conventional spanner is jamming. Is there a special plug spanner required or will I just have to force the issue? Any constructive help appreciated . . .