The parking brake on his '94 GS doesn't work very well, even when the pedal is fully depressed it doesn't hold the car.

Despite my Dad's knowledge of cars he can't work it out.

Can it be adjusted? If so, how?

He's sorted it.

If you take the rear brake discs off, the parking brake shoes and adjuster are behind them.

Thats handy to know, always thought there was a tensioner under the back of the motor.

Despite adjusting my parking brake a few months ago it is already loose again and now requires 8 clicks to hold the car on our inclined driveway, which is basically the pedal against the footrest.

Does this mean I need new cables/parking break or should we try adjusting it again?

In theory, the Shoes should never wear because they are used solely when the car is stationary.
If you need new shoes then you can get aftermarket items. They probably are worn due to it being so easy to drive off with the brake on. I never ever use mine unless I park on a big hill and even then, I will always look for flat ground to park on and walk that little bit further.