Many thanks for your advice. I took off the motor and found the linkage assembly moves freely at all the ball joints and central pivot, but the two end pivots (which attach to the wiper arms) are extremely stiff and will only rotate through about 80 degrees when considerable force is applied using mole grips on each end. No wonder the motor was getting hot! You were spot on about that, and the 13 mm nut :-) The two end pivot pins and the central pivot pin are all held in by a circlip and washer. After removing the circlips and washers, the central pivot pin slides out of its sleeve easily, but the two end pivot pins will not budge from their sleeves (which are pretty corroded) even though they do rotate a little within them. If I cannot get more movement in these end pins, I think I will have to get a new linkage assembly, and hope the motor is still ok. Have applied WD40 and will see tomorrow. Thanks again for advice - greatly appreciated :-)

Final post on this one: the old motor worked fine when I removed it from the chassis and tested it (by connecting it back onto its socket and holding it while getting a helper to switch the wipers on and off again), even though it had become extremenly hot as mentioned in previous post. The only actual problem was that both of the wiper arm spindles were almost impossible to rotate, due to corrosion I suppose. The linkage arms were okay and so I just needed the chassis part which incorporates the two spindles, which I ordered online from autodoc, total cost about £85. The complete assembly from eurocarparts costs £370. It took about 1 hour to swap over the motor and linkage arms, and about 1.5 hours to reinstall the complete assembly. As mentioned in the Haynes manual, it's important to ensure the motor arm is in the correct position when reassembling. The installation into the vehicle was rather tricky as the complete assembly is rather unwieldy and has to fit into a restricted space, but patience is the keyword. The Haynes manual was helpful. I did not disconnect the battery, but simply removed the wiper motor fuse to isolate the circuit.