Since nobody else has answered, here is my penny's worth. (I will be giving the Tourer an oil change myself once I get a couple of days clear)
Buy two litres only, but drain the oil from the gearbox into a really clean bucket (one of those washing up bowls from the poundshop for example). Leave the gearbox drain bolt off overnight to make sure you get as much out before replugging. Then fill the gearbox with the 2 litres of new stuff, and pour the old fluid into the bottles until full.
Whatever is left should be the dirtiest part with all the bits in, but you should be able to measure by pouring into another old container, or guessing.
If there is none left over then you know only 2 litres has been taken out, if some residue remains then you could strain whatever is in one of the bottles and pour in to the same level, or buy another litre if unaccpetable. I doubt your car will care about 5, or even 10% of old oil in amongst the new stuff.