About 6 months ago the gearchange on my GT JTS manual started acting up. Lateral movement of the gearstick was stiff; I often couldn't "reach" 5th gear when driving. The gearstick failed to self-centre and finding 4th gear in particular was a problem (when I failed to reach 5th I usually then couldn't find 4th and had to put it in 3rd without lifting the clutch pedal to get me on the track to 4th). I could see how the excess effort I had to use on the gearstick could cause further damage, and the absolute nightmare of doing the three gear shuffle to end up remaining in 4th at 70 or more mph was totally unacceptable so I resolved to do the "top hat" fix, or maybe even, if necessary, to change the cables.
As a prelude before I even sourced the replacement parts I had a "ferret around" to see if it would be within my compass or if it should go to Gareth at Northern Alfa. Without removing any parts at all, with the engine off, I got my son to change through the gears so I could see the operation of the cables. I noticed that quite a bit of "crud" had amassed around the inner cables for both lateral and back and forth operation - but the lateral one was by far the worst. I cleaned this out as best I could - leaving all parts in situ. (Scraping and brushing and spraying with WD40). Just by doing this the gearstick began to make a start at self-centring. I figured I was on to something. Concentrating on the lateral cable I continued to clean (spray with WD40 - work the gearstick side to side - clean off the black crud that exits - and repeat) with each repetition the gearstick self-centring improved. Once I'd removed all the crud that wanted to come out by this method I applied a little grease to the inner cable ( I suppose silicon grease would have been best for this - but I just used what I happened to have in the garage) and worked that as far as I could into the cable (by moving the gearstick side to side and "feeding" the cable with grease).
The improvement was remarkable, although I expected it to be only short-lived - but it is six months now and the gearchange is still very much improved. I suppose I could make it even better by replacing the top-hat bushes - but the gearchange is fine so I'll leave it for now.