Have you ever drunk your shake, not washed the cup/shaker up straight away, and then caught the whiff when you do wash it up? Thats why you dont pre-mix it. It smells rank, like its gone off!
When out and about with work, I take my shaker and a hand held whisk (just a whisk, not powered at all), put in half the water, mix it with the whisk, add the rest of the water and shake. Its fine, problem solved.
Got my shaker from the pharmacy I get LT from. Although they are a generic design, lots of places sell them, including Tescos.