Is it possible to change the input shaft seal in position rather than buggering up the alignment pulling out the rack, If so can anyone tell me what dimensions the seal is to replace it?

It is possible, I did it recently. It is a bit tricky though as you have to remove the steering shaft and all the pipes going to the valve body area of the rack. There is also a bearing behind the seal which has to be removed to get the seal out. I do not know the seal size, I took the old seal down to the local bearing place and they matched it up for me. Cost about $4.50. I also had to make a new gasket for where the housing bolts on to the rack. For this I got some cork based gasket material from Super Cheap, 1.2 mm thickness I think.

Is it possible to change the input shaft seal in position rather than buggering up the alignment pulling out the rack, If so can anyone tell me what dimensions the seal is to replace it?

Taking the rack out of the car won't stuff up the alignment. If the seal has been leaking, it all will need a good clean anyway. The seal probably leaks because there is lateral play in the shaft due to wear, and needs setting up correctly so the seal will last more than a few weeks.

I was thinking age of the seal was a more likely cause. I have owned four magnas and only the last two have leaked on the rack. A lot of gravel road driving wouldn't help and the roads are a lot rougher now than they were in '96.

MadMax makes a good point but I think his scenario is unlikely as the bearing supporting the shaft at that end is just under the seal. This bearing is a fairly sturdy roller bearing and the load on it is not high but you can check it if you pull the top off the rack either in or out of the car. My vehicle suffered from poor servicing before I bought it and the PS fluid was black.

Does the input shaft have a key or a spline in the coupling under the locking bolt?

The shaft is splined but it has no master spline or keyway so the coupling can go
on in any position. I scribed a line in the housing that lined up with the slot in the
coupling to give me a reference for reassembly.