i just failed on mine...and it wasnt even obscuring the lettering...the mark was under the W, so it was still totally legible
if yours has, say a P and it now looks like an R then you wil fail...the guy in mine cited speed cameras etc
it wasnt a scam because the garage dont make them.
and they restest for free if its in a few days.
i got one for £8... from a local place, rather than halfords.
you need your licence and log book to do it