Yes they will Geoff.
If the letters and numbers are not of the standard required size, or they have been altered to spell out names etc, then they will pull you just for that, give you a fine for displaying an illegal plate and send you on your way.
They do it a lot round here - especially on a sunday.
But i agree that there are far too many bluddy cars with illegal plates - hundreds of them near me spelling "Abdul", "Khan" or other such names.
Annoys the hell out of me it does.