[Arran]

First rule about touching up an old finish, if it has scratches or worn spots but isn't flaking you may be able to do something with it. If it's lifting, flaking off, or it has large areas missing, the finish is dead and there isn't much you can do but strip it and start over. Stay away from Formby's or other "refinishers" no matter what you do, all that is is solvent that melts the lacquer so you can smear it around, but it also smears the shading lacquer around making a muddy mess.
There was a guy on the alternative forum that was desperately trying to "restore" the original finish on a brand Z console with cowards restore a finish. The trouble was about half of the lacquer was gone from it and it had clearly been in a damp basement along with the lifting veneer problems that go with it. Well the peanut gallery was telling him what a great job he was doing and that it didn't really need to be stripped and refinished. The problem is that like a tung oil finish you need to keep adding the cowards to it every few months because it dries out again, there is nothing to seal it in.
Regards
Arran