Thanks for all the replies - I didn't expect this would generate so much interest!
I do have a drain plug at the bottom. This is just routine maintenance - the unit is working perfectly. I just wanted to know if it was really necessary to pump the new oil in from the bottom as the last time this was done the boatyard insisted that is was necessary to avoid getting trapped air in it. I don't have any equipment to do this. I'm convinced however that as long as I keep the drain open when refilling from the top there should be no trapped air and I can close the drain as soon as the new oil starts to come out. This is my theory anyway!