Sounds like gearbox problem if the lever is jumping back out . Your making sure the clutch pedal isn't obstructed by anything like the floor mats getting in way of pedal ?

 

Yeah there is no problem whatsoever with any other gears, only 3rd? That's what was making me think it was something in the gearbox instead of anything to do with the clutch as its specifically just one gear.

 

Sounds like a badly worn selector fork.
Is there a lot of free play on the gear selector mechanism when putting it into gear, particularly 3rd gear?

 

There's a little play between gears but nothing major, well not enough to make me question if there was anything wrong before now.

 

If you'd said its not going into 3rd without some persuading I'd have said worn gear lever linkage / cables etc , but has its jumping out of gear I'd suspect the gearbox.

 

But a worn selector fork can cause the gear to jump out, which is normally shown up in slightly excessive movement in the gear selector movement. The gear selector forks don't just move the synchro hubs they stabilize the gears by keeping them running in the engaged position. Wear of just the one gear is typically caused by a driver who rests his hand on the gearstick when driving, putting an unnatural load on the selector fork.
Would be worth checking the links / cables are adjusted correctly before condemning the gearbox though.

 

Might light up some things, and ring bells, is this a 5 gear or 6 gear box? What engine does this astra have.

 

It's a 6 gear box and it's a 1.9 CDTI

 

M32 then pfff chocolate box i am afraid, i would start asking around how much is to fix, i would expect a couple of hundreds... But ask at garages specualized in fixing and reconditioning gearbox otherwise you will end with a swaping gearbox bill which i recon is not the case. M32 boxes are known for butter bearings and other parts, probably you will find that fift is not quite ok also... Hardest part part will be locating who can fix that around you... Obviously if you leave it will get worse and you will end up needing a whole gearbox.