The plumber said he had connected the replacement immersion heater when I called by to see how the job was going. 


I decided it was best to replace this with a SFCU as the immersion is connected to a socket ring circuit and only a backup for if there's a problem with the gas boiler. 


Interestingly I put the RCD tester on the immersion terminals to trip the RCD on the consumer unit to try it before disconnecting the original wiring and this controlled blocked the RCD.


Andy