Heat the white sticky residue up with a hair dryer and remove it the best you can peeling it off. Autoglym Tar and Adhesive remover on the rest. Took me ages to do mine.
Also heat the whole plate up first and it hopefully should not leave much residue when you remove it.
a dab of hydrochloric acid removed it on mine
I won't be going anywhere near my car with acid .. pmsl I had a bad enough experience with a touch up pen lol
White spirit got it off mine no prob, and scrape it with your finger nail (personally wouldnt use a teaspoon) lol
petrol or meths