Hi Colzie,
the gearbox in your car is a NCVT. they work off a electromagnetic clutch system. The clutch unit is filled with iron fillings, when you put into gear the contacts energize the clutch and the fillings are magnatized and the clutch is engaged. (technical stuff)
there are not many things you can check yourself, you have done just about all you can with checking the oil etc.
Is the gear lever hard to move into gear? (if so your clutch may be worn out)
Is the gearbox warning lamp on?
It could just be that the contacts on the clutch unit have corroded and not making good contact.
if you wont to check this you will need to remove the contacts. this can be done easy.
open the bonnet, at the front of the gearbox you will see a plate with 2 wires and 2 bolts. undo the 2 10mm bolts and unplug the wire connector. remove the contacts and check there condition. than you can see inside the gearbox and check the clutch contacts also.
hope that makes some sence.