Same gearbox as mine so that's easy!
With a warm engine/gearbox, jack the front of the car up and remove the two under-trays. Hopefully, you can identify the gearbox from here, and locate the drain plug underneath and the fill plug on the right hand side (both a hex key bolt but I don't recall the size). UNDO THE FILL PLUG FIRST! If you drain then can't fill then you're screwed! Now drain just as you would for engine oil so leave it draining and go and have a brew. In your case, have a good look in the drained oil for anything that could be causing you problems.
Now for the filling (after refitting the drain plug, ensuring it's clean). My first attempt at this was messy to say the least! As it's a side fill it's a little tricky getting the oil in there. By the time I came to my second attempt, I'd had a brainwave. Get one of these
5L LITRE MANUAL KNAPSACK GARDEN PRESSURE SPRAYER SPRAY KILL WEEDS | eBay (available from the likes of wilkinsons and the range too) and remove the spray handle leaving just the rubber hose. You'll find that this hose fit nicely into the gearbox. Do this by feeding the hose down from the engine bay so your then not working underneath. Drop the car back onto LEVEL ground and put a small amount of the castrol oil into the sprayer (check it's free of any crap before starting) and start pumping. Hopefully, the oil should start transferring into the gearbox. If any is dripping out, you need to feed the rubber hose further into the gearbox and try again.
The stated capacity of the gearbox is 3 litres of oil which is, essentially, fill to spill. Once full, jack up again and refit the fill plug. Both the fill plug and drain plug should be tightened to 45Nm, which I found to be approximately 4 white knuckles
. You should then take the car for a quick spin then check the level again ie level with housing at the back of the fill hole (difficult to see but the photo shows what you're looking for and why fill to spill is the way forward...you can't see the oil because of the tricky access!).
I have yet to get round to doing this part yet but will be checking it in a few weeks while doing some other work, but the gearbox has felt good ever since.
That's it! Now I'm not saying this will cure your problem but, seeing as a replacement gearbox will be brewster's, approx £40 doing an oil change is definitely worth a shot!
Good Luck!
