ORIGINAL: Johnny C
The number plate is larger. Mine is stuck onto the plinth.
I had the same problem, number plate kept falling off after I parked into a space at PC World a little overenthusiastically and bashed the plinth on a bollard. To secure a new one, I ignored the usual screws from Halfords, I used some plasterboard fixings - wide plastic cylinders with a central hole to take a screw, these screw into plasterboard with a wide thread, they went into the bumper fairly easily with a little drilling (the old holes didn't match up with the new plinth) and I used some silicon sealant to make sure they stayed in and to stop them rotating as I screws the outer screws in. You then screw some ordinary wood screws through the plinth into the fixings and glue the number plate onto the plinth.
They've not come out, and it's fairly solid.
Thanks for the feedback, much appreciated. As it happens, the wide plastic cylinders have popped out of the bumper and are atached to the screws in the plinth. I assume from what you are saying then that some silicone should do it - I can then pop the old plinth back on the bumper?!