Okay. Then basically any immersion element of the correct length with the correct boss size (looks like 1.5inch) will do.
Just make sure it has NO copper bits like pockets as they corrode much faster than the incalloy parts.
One should NOT use std immersions on a stainless cylinder.
HTH
I was going to recommend contacting The Electric Water Heating company at Harlow I think they only do Internet now they used to have a store in East Ham E13 and were always a great source of info ....glad you got it sorted......regards Turnpin