I turned my attention to my particular unloved GD Turbo last night and after a fair bit of grief it's all working again now. As a particular poor fettler who bought the GD because it can be fixed with a hammer and grease here's what I learnt
1. It's easy to take the post to bits and clean it up.
2. If it still doesn't slide well, disengage the pin bit (unscrew it) and if it now slides well it's the pin preventing the post from moving. This may be because the thumbscrew that the cable uses to attach to the pin may have begun to unscrew. If this is the case then no amount of cable tension will fully disengage the pin. You won't know it's begun to unscrew until you detach the cable from the thumb trigger to access the pin spring mechanism at which point you'll be refitting a new cable.
3. When you begin to service the cable or the trigger end make sure you have the correct (easy to misplace) hex tool (4/40??) to unscrew the cable from the thumb trigger. It's an odd size that you won't have in a standard set or multitool and you should've received one with the kit. If not go and get one as you're nacked without one. You obviously also need a new gear cable too.
That is all. Anyone else got any tips for the maintenance of this piece of IKB-era engineering?