Julian,
I am by no means a grass casting expert, I did it for the first time seriously at Marlow Bumpus' Spey Days this past April. While it is casting, it is different from casting on the water. As Mean Mr. Mustard alludes to, it is all about getting enough stick to load your rod for the cast.
While I did manage to launch a few satisfying casts in the competition, those who had the technique of properly loading the rod on the lawn did it better. This is what concerns me about practicing exclusively on grass. There are definitely aspects of the cast that are the same, but if you used the grass technique on the water you would probably have a hard time getting your line "unstuck" as you would be laying far too much of it down on the much "stickier" water.
Not that I think grass casting is useless - far from it it. I just suggest that you try to cast on water as well, even if it is a pond. Still water is not as pleasant to cast on as moving water, but the "stick" that water provides is what you will be working with when you are fishing. I prefer to practice on moving water, but realistically I do my practicing on still water - it is closer to home.
:devil: however, Ed - I will be practicing my grass casting technique for next April :hehe: