Buying lobster- Cooked lobster, approx. 400g- presumably thats net wt. NOT approx. weight of meat, correct?

I'm afraid it's wishful thinking to expect the 400g to be the weight of the meat. The head is inedible and accounts for a large part of the weight, as do the shell and claws, the latter being edible. The flavour may well be sublime but so far as value for money goes you'd be better buying from a local fishmonger.