Mix together the butter, garlic, lemon juice and herbs and season. Place in a sheet of clingfilm, roll into a cylinder and twist the ends to seal. Put in the freezer for 5 minutes to harden a little.
Snap the claws away from the lobsters and crack the shell with the back of a heavy knife to remove the meat inside. Cut each lobster body in half lengthways. Wash out the head cavities with cold water and divide the claw meat between them.
Put the lobsters on a grill pan, cut side up, slice the butter and place on top. Cook under a preheated hot grill for 5–7 minutes until bubbling. Serve with green salad and potato wedges.