why not connect the least used spur to the MCB feeding the next to least used spur, if the rating is low enough to protect each, then connect the immersion heater to the freed up MCB, or connect the two least used spurs as a ring circuit. You can use a switched spur to fuse and switch the immersion heater
-------------------- Derbyshire born, Derbyshire bred. Strong in the arm, Weak in the head.
Happy sailing.
Malc