I have an old GPO jack plug fitted onto a standard wall box. I want
to replace it with an up-to-date socket so that I can use it. I
haven't been able to find its details online so if anyone can help,
I'd be grateful.

It was installed when the house was built, about 1970. It has five
terminals:

1 - brown wires (2)
2 - orange wires (2)
3 - blue wires (2)
4 - green (1)
4A - brown which is looped back to 1.

There are two cream-insulated wires coming into the box, since this
extension oulet is one of several in the house and the wiring goes on
to the next room.

The fitting is flush to the wall with a standard metal box behind it,
set into the wall exactly as a light switch would be.

The cream plastic face of the jack has "GPO" on it.

What is its model number and what which wires would I use when I
replace it with a standard extension socket?
Ta,