Well.. we're getting somewhere. If the beast runs once it has started, then it pretty much rules out most of the other potential problems above. It's just a case of cranking the beast over and it will start.. by the sound of it.
Measure the battery voltage. It should be 12.6 volts. If it's less than that, top up the levels with distilled water (while you're there..) and then stick it on a charger for 10 hours or so. That ought to be plenty.
When you finish charging it, the voltage should read 14v... it'll gradually drop down to 12.6v. If it goes any lower than that (e.g. the next day) then either the battery is knacked or you have a serious drain going on. I'm assuming we'll leave the battery in the car for the time being.. removing it might just upset the immobiliser..
Anyway, each cell makes 2.x volts.. so 5 cells (if one is dead) can still give you a reading of 11.x volts. It's not enough. It has to be 12.6v.
If the battery is all happy, around 14 volts and fizzing away when you finished charging it, then it should turn the engine. If it's sluggish, then the battery could still be goosed. It can make the volts but not have the ampage needed to activate the solenoid.
If the engine makes no attempt at all
to turn over, then the problem could be the starter or starter solenoid. The clicking (starter relay) is good but the next bit in the link might not be doing it's thing.
Changing the starter is a bit of a job.. so easiest way to check it's okay is just to stick a different, good battery in there just to try again. If you take the one out of a Ute, then that'll have enough juice in it.
If that doesn't do it, then you're looking at the starter or solenoid .. but if the different battery makes all the difference, then you just want a new battery.
If you buy a new battery, get the biggest barst' that will fit in the tray. I think an 096 (?) as fitted to the GTV 3.0Ltr will fit. It's got a few more amps..
Of course then you must check why the battery went so flat. It could have just died (a cell plate broke off etc. which would be pretty fatal) .. or you may just have a current drain .. but anyway, let's get the beast started first.