you need to purchase a new thermostat for the shower. This will cost about £50 or so from shower-warehouse. The reason it gets hotter when you turn on the basin tap is because less cold water is getting to the shower . You can change the thermostat fairly easily by prising off the chrome disk with the Royale name on it, undo the screw, pull off the lever and then use a large spanner to undo the chrome body. You can then extract the thermostat which is about 4cm long, half cm in diameter and has a rubber seal on one end. Good Luck
gerry, October 2008