Well, what do you know, driving to work this morning overdrive worked and stayed on for the whole highway commute at ~95kmph although as soon as I hit 100kmph it disengaged, but worked fine again at ~95kmph. Off the highway though, in a 70 zone, it would not work. Wouldn't work on the drive home at all, either (absolutely nothing happening). Had another quick look and wiring is the same as I left it.
Thanks for the advice guys. I appreciate all the help I can get as I'm not as techie as I sometimes make out!
Where is, and is there easy access to the gearbox switch? I'd like to try the solenoid test if I can get at the switch. As for 'clicking', on the road it's random (regardless of speed/temperature); clicks and engages (sometimes disengages a second or seconds later), clicks and doesn't engage, or doesn't even click at all. I don't have axle stands yet so getting under the car isn't possible for me, yet... Same goes for the fluid change, for the mean time anyway.
To be honest, I'd like to say overdrive is speed dependent (especially after my test last night) but after today, and thinking back, whether it works or not is just, so, so random. I really think it's something as stupid as two wires touching each other somewhere (depending on general movement of things) but I don't know.
If it's a matter of pulling the box to get inside, that's a bit far fetched for me to be honest - if that's the case I'll have to live with it!! I think I read online about the filter sometimes being a suspect.