Colin, I am wondering if someone could dig up the appropriate bit of Martin Cleeve for you. It is in Model Engineer but whether it is in the later set of his articles, I don't know but I doubt that it is in the ones which start Vol 113. I am in the middle of a Super 7 rebuild but three things are immediately apparent on your description. The first is that your bed top is worn. This is a cheap and easy improvement to do a regrind. Remember the wear which is causing the trouble is in two planes. The dip in the bed at 6" from the headstock and 6" in from the front shear. The Saddle goes into this "narrow" Theoretically, more than the top thous of wear disappear. The next point is to square the wear of the front shear to go parallel with No 4. I am ignoring the under-wear(sorry) of the shears. To all intents and purposes, the lathe bed is then fine- or within most peoples aspirations. ( You are never going to get a new lathe- new lathe- so accept what you have)
The saddle has a tongue- no I forget the proper word- but it will be worn down at each end. Call it bowed! You can scrape it to remove the bowing- but it could be out of your present range of capabilities. You can soon check what I say by blueing up the tongue- and see what I mean.
Unless you bring the alignment of the bowed tongue to be parallel with the headstock, you might as well go home because it will never be a success.
You therefore bear onto the front and rear shear and make the tongue a fresh air fit. The rear gib was the fresh air one before- but should be nigh perfect.
Importantly, I am not talking theory, Cleeve did, Myford has done it and I have done it twice.
I am not an engineer but I do understand geometry fairly well.
The important thing in all of this is that you will not spoil anything. All that you will be doing is drilling and papping four or five holes for gib svre adjusters and adding a gib.
It can all be made to revert but there is a bonus.
The modification provides for the re- alignment of the saddle with the headstock.
I hope that these modifications are easy and acceptable.
Norman