Split the tails then put them on the barbie shell side down, then sprinkle a bit of butter and garlic onto the flesh. I cook them for 10 minutes or so, NEVER flipping the flesh onto the hot plate. This way the juices stay in. With about 2 minutes to go, I'll throw some beer or white wine onto the tails, and quickly cover them with a stainless bowl. This steams them from above, and seals all the goodness in!
Steaming them is better than boiling them if you choose to go down that path.
The following is some comments from a chef about cooking crays.
"I found that the best way to cook crays is to steam them as it does keep in the natural juices, personally, I love the unadulterated flavour of most seafood not the least crays, bugs and prawns. As a chef I have had to prepare crays in many different ways and having read a lot of other members posts I found that there is a lot of great variety with which to serve crays and they are all mouth watering. I think that I can say without a doubt that there is nothing worse than to ruin a good cray by way of overcooking it... it is such an expensive waste. Having cooked crays professionally I found that the safest way to prepare them is to understand the fundamental rules and that is ( for steaming ) eleven minutes per kilo at 95 degrees with 100 percent humidity... taken out of the live tank and placed into fresh water for approx half an hour to kill them.. then placed into a tray... we mixed up 2 cups of white vinegar to half a cup of raw sugar and gently poured this brine over them... finally spoon some of the residue sugar left over at the bottom of the brine mix over the cray and place them in the steamer... it is important to be fairly precise with the calculations of it all... at home I have done this successfully using a bamboo steamer over an electric wok filled with water. I have also ruined a couple of good crays by boiling them too long... the trick is to get them out of the cooking process at the right time and as one member correctly commented... put them straight into ice-water for a good half hour to stop them from continuing to cook... I hope that this has been of some help."
I found 11 minutes per kg was insufficient to fully cook the flesh in the body of the cray using a home steamer. Perhaps commercial steamers are more efficient.