Thanks everyone. I am sticking with it!
I use a hand blender to mix the shakes as I hate any lumps, & I make them fairly thick so they are gone quicker.
I have a blender so once I dig out the ice tray & have some ice, I'll give it a go that way, I have to say the colder the shakes are the easier they seem to be to drink, so doing it this way makes sense.