I have moved into an old property and one of the bedroom lights has been disconnected. The wires are still there but I want to connect a new light fitting to this.

There is presently two cables coming through the roof each containing a red wire and a black wire, the red wires are taped together. There is also two bare twisted wires there.

Can you please tell me how to connect them into a simple ceiling fitting containing one bulb.

The bare is your ground, and will be good to determine which is hot when switch is on, then identify the other black with white tape, this will be your neutral. Leave the reds together and don't touch.
Then connect white to white, black to black and ground to grounds.
Turn power off.