Unfortunately unplugging the 'speed control module' or 'speed sensitive control relay' (under the drivers foot well) won't prove anything. Of course the steering will get very heavy because it will go into limp mode.
If your getting sudden problems with your steering Steve then its not going to be fluid. Power steering fluid doesn't suddenly decide one day that it is going to make your steering stiff. The way to check your fluid is simple. Dip your finger in the reservoir and see what colour it is. It should be red and not brown. then smell your finger. If it smells burnt then it time for a fluid change. You will see air bubbles...this is normal and nothing to worry about.
The likely problem with your steering is almost certainly the Speed sensitive control relay. It's a common fault and i had to replace mine last year. It costs about £85 from the dealer. Called a speed control module, it lives above the brake pedal anchored by a very strong metal spring clip with very sharp teeth. Be careful! I found mine had moved and was jammed against a bulk head bolt causing damage to the head of the relay not visible until you get it out.
Symptoms are intermittant loss of steering control resulting in the steering feeling heavy. It usually resets when turning off the ignition but comes back sooner or later. Eventually it will fail alltogether and driving will not be easy. When i replaced mine it made a huge difference.
I have a thread on here about this....
http://www.vpcuk.org/forums/showthre...itive+steering
Changing the steering fluid will not cure your problem.