With the car on level ground (once you get the acoustic pan off, three 10mm bolts at the front and two 8mm bolts behind the leading wheels) and the gearbox plug removed (usually a square socket drive) the oil level should be just at the lower edge of the plug on these models. The gearboxes are usually pretty reliable on these models, same with the gear linkage which you can check for wear and tear whilst you are under there. The symptoms sound very much like the clutch assembly is falling to bits and not allowing the clutch plate to rise on command. Is there also a tinkle tinkle sound with no load on the running engine, if so it sounds like the thrust bearings are at the end of their life, the sound should change as you push the clutch pedal in and out.

One other tip, non engine related, referring to the rear tail hatch. The lock mech is a combination metal/plastic affair and rainwater has a nasty habit of seeping through the lock barrel and rusting the metal acutator arm to the point where it sticks. First you know is you are driving along and the tail hatch pops open. Easy fix, remove the lock mech, knock out the roll pin to release the actuator arm, clean up, regrease, refit and you'll never have to touch again.

Despite the picture the press have painted over the years the Rover 200/25 cars are decently built and if looked after will give years of good service.