my old FXSTC won't kick over, battery is good, can hear and feel relay clicking, 

where can I start to fault find, would it be the starter motor or something.

Thanks in advance, regards Mark

Relay clicking indicates its getting power to that point. 

Put a metal ruler or screw driver on your starter and see id ur starter motor magnetises when you hit starter button.

If no, you know its not getting powered and ur issue is before that point.

If it is magnetized, it indicates its powered. Tap it with a hammer or mallet. Could be sticky solenoid.

Battery has been on charge and is showing 12 volts,

tried the starter tapping, to no avail,

What am i looking for with screwdriver (where do i put it, am I listening wit it?)

Sometimes batteries will show 12v and will drop to half or less as soon as its put under load. See what voltage ur battery drops to when you hit the starter.

As for screwdriver or steel ruler, you need to put it on the starter motor from carb side. Any where on the body is fine. As soon as you hit the starter button that whole starter motor should be magnetised. Use the ruler to screwdriver to see if its magnetised WHILE you are pressing the starter button.

Battery only drops to 11.7v when I try to start it

Early run bendex type later run Nippon denso from your description it 

Sounds as if the contacts in the solenoid need a clean or replacement 

Which you can do with out  removing starter from Bike  that is if it is a 

Nippon denso starter/ style   (Hilux, landcruiser parts )The solenoid is the 3 bolt cover on the starter Make sure you disconnect the battery earth before you investigate