As Alan says, inserting a scalpel blade between surround and numberplate and twisting should ping it off.
However!
I ordered new plates from Light railway stores, and asked for the "Dapol" size, which is a bit smaller to fit within the surround.
I have used these for the 14XX and they are a nice close fit within the moulded surround.
The pannier surround seems slightly larger, and there is a slight gap, as can be seen in this photo.
I have now ordered "standard" sized plates, which hopefully will be a better fit.
