"Top hat" is a term that most alfa people will understand. They are the bushes that the gear selector runs in on top of the gearbox. You take the battery out, and the selector is visible. Unto the 17mm nut, pull the shaft out, clean up the bushing (it will be full of ally corrosion chuff), put the new bushes in with a load of grease, all done. Very simple job, and very satisfying.
The bushes tend to affect side to side movement of the lever. My experience was that when cold, it was hard to get into 1/2 and 5/6/R - as the car warmed up, it became easier.
if your problem is inability to select 5/6 when warm, that is really pointing to the gear lever carrier - the £15 bit of plastic that the gear lever sits in.