I would not place too much weight on the Halfords recommendation myself.
Their battery listing for a 2000 model year Defender is wrong (too small) and the listing for a Plus 8 is the 4/4 (smaller capacity) type.
My feeling is they tend to list things they stock, or have a higher profit margin on.
NGK publish or did a proper book (mine is 1990!) and any decent factor should have a copy and will probably let you peruse it.
In this case, Halfords look to be correct as a plug should occupy all the threads in the head.
