The single feed to the immersions could perhaps be used as is by installing a Horstmann Economy 7 type controller, fed from one of the existing FCU's, which could output to the two immersions, as it ensures only one can be powered at any one time. So setting the timeclock to E10 periods would power the bottom immersion on cheap rate, and the rest of the time the boost button would be available for a top-up, on peak rate of course.
But would the pump draw too much to be left on that citcuit as well?