Chris,
Some MBs have the rear plate stuck into a black plastic surround which itself is screwed to the rear panel. If yours is like this and there are no obvious screws on the face you can,.....with a bit of care, lever out the plate from the surround....there will only be a few tabs holding it in.
Be cautious not to damage the surround if you use a knife or screwdriver and as you begin to prise away try flexing it away rather than just ripping it of.....which might end in complete failure...
ATB