Your Car Clinic Q&A on the removal of spark plugs (November 2013 issue) refers. I have always found the following procedure effective, whatever the type of plug. First, apply some penetrating oil to the plug thread and allow it to soak for a few minutes. Then using a long-reach plug spanner and tommy bar (that is, not a cheap box spanner), apply a tapping motion backwards and forwards until the plug loosens. If it obstinately refuses to budge, keep knocking it – two strikes anticlockwise and one clockwise – until it moves.
Bill Dallas (via e-mail)