No problem Tim. Sounds like swapping for the 22l might be a good idea, be much quicker to heat. I went for twin coil as doesn't cost much extra and means it is more versatile - room for adding additional heat source if desired. Also I think having the coils in series will improve heat up time with the engine considerably as it is effectively doubling the surface area of the heat exchanger.
This is the valve used to turn supply on/off:
Solenoid Operated ON/OFF Heater Valve - Car Builder Solutions - Kit Car Parts and Accessories
It is mounted just after the tee from the heater supply hose under the bonnet. I have it wired via a switch in the cab which is supplied from a 12v ignition live.
I have an expansion vessel and accumulator installed on the fresh water plumbing side. The expansion vessel needs to be plumbed in downstream of the calorifier hot water take off. I had to adjust the pressure in mine as it was initially too high meaning the pressure relief valve was opening rather than the expansion vessel doing its job. This will depend on your water pump pressure. The accumulator is fitted just after the water pump outlet although could be installed anywhere on the primary cold water feed. Your water pump type will determine whether this is necessary. If it is a basic pressure switch type then the accumulator smooths out pressure spikes and stops any hunting from the pump.
The 230v immersion heater is wired into the consumer unit via a 5A MCB. Again this seems to work well.
One thing you may need to think about mounting it under the floor is thermo syphoning. I have heard of this problem in boat installations where the calorifier sits below the engine block and as the engine cools overnight it causes a thermo syphon - effectively cooling the calorifier hot water prematurely. My coolant hoses run below the engine then up to the calofifier which gives the horizontal hose run as the lowest point in the system and there hasn't been any problem with thermo syphon.