MooMoo as John mentions the body of your plug wrench is wider inthe body than the wrench end if you look at the photo I posted the body and wrench end are the same size,
what Halfords have sold you looks like a 'car type' plug spanner down sized to fit bike plugs, If it helps I can email you a larger photo of the spanner I use so you can compare it against any you plan to buy.
HTH
PS. I normally find Halfords quite good at taking bits back even if you bought the correct part but it doesn't do the job.
The original problem was that I didn't know what size I needed.
Now I do, thanks to those who helped.
I know your the font of all knowledge Steptoe regarding the GS, I'm sorry that I have burdened you in your need to post the above.