Sorry my post wasn't very clear, was rushing this morning !
We're at 84,000 miles and the service list says the oil should be replaced,
We're going to France in a few weeks and have checked the oil it's at the bottom end of 75 on the stick
So was wondering do I have to replace it or can I just top it up,
There are no leaks etc and car is running fine
Replacing the oil doesn't take long. It holds 7.5 litres of Dexron II fluid. There is a drain plug at the bottom of the gearbox on the bellhousing, a plate covering the ring gear/ flywheel has to removed first though (only 3 bolts). The access to the rather large drain plug is easy. If possible do it on a flat surface of not all the oil will drain. It's easier to refill with the air intake pipe removed as it will give a straight run to the dipstick tube.
If you have the time before you go I would strongly recommend changing the oil. These gearboxes are a fickle thing and are expensive to repair, trust me, my wife's has just gone belly up and I'm in the process of trying to find out why.