I feel your pain mate, All this year has been unlucky for me too, one thing after another. If ur old man has taken the car in to have the leaky seal fixed and it's still leaking, I recommend you take it straight back. As for gearbox oil, yes you can topup the oil in the box, make sure there's always enough oil in there. I've lost a box just sue to the lack of it. Grab ur haynes and locate the filler for it. Make sure you take the car back to the garage u had it fixed at and get em to fix it properly.