If you get your hose to a fine but strong spray a simple jet of water is enough to kill most aphids. I take the rose off the end of my hose and just put my thumb over it so it's strong enough to wash off (and kill) the aphids but not so strong it makes holes in the leaves.

If that doesn't work, try getting some lacewing larvae for your greenhouse. They consume hundred of the blighters every day and will hopefully breed and give you a lacewing army in your garden year after year.

If you google 'lacewing larvae' there are a few people who sell them. I think it's around £15 for 500 but that might mean you never have to worry about aphids again 'cos nature's doing the hard work for you.