"get the pride and joy in" - it's the 'in' that's the problem. When it lives on the street, in cold wet England, non-essential tinkering must wait till we see the sun again.
There is a threaded hole in the drum, and if you still have the little screws which hold the drum in place, remove them, screw one into the threaded hole, and it will force the drum off. You still need to get at the opposite side and help it off with a long lever. When you put the drum back...
If you look at the picture, Rattlegun's landy has larger round rear lights, with a clear section below. The black light like you have is additional on his.
Does anyone know: you don't need an MoT because of the vehicle's age; you take it for an MoT for peace of mind; it fails on, say, one number plate light u/s. Is it then off the road, or can you continue to drive it, and just replace the bulb?