Spruce , this is what made me think of trying my own , but I see that you have already been there and done it , looks like too much of a headache to me 
But i see that this guys shut down at temp too , although not completely it seems , who knows ?
https://www.youtube.com/watch?v=qcfPMQw3toU
I saw that a while back. Unfortunately those guys were so busy talking rubbish beating their chests in triumph that they never noticed the unit going into half heat mode. Even although the heater goes into shut down mode the water pump still continues to circulate water through the system. Its on standby mode. The pump will only stop circulating water when the unit is switched off.
When they added cold water to the bucket the boiler went into power up mode when the water's temperature dropped to 69 degrees C. Now when window cleaning, we would zap most of the heat from that bucket of water before the boiler had completely powered up again.
And this is when a small boiler would battle. You will be zapping all the heat from the boiler and the boiler just isn't powerful enough to put in extra to add heat back into reserve.
The Peugeot 807 heater is the same heater that fits the Citroen C8 as it is the same vehicle. The problem with that heater is that it sits under the car and collects all the road muck off the wheels. The Rover 75 heater mounts up in the engine bay and is in a much better environment.
Eberspacher has a new 12kw hydronic diesel heater. That could be an interesting option as it doesn't use that much more fuel than the 9kw.
If I had the energy I would like to fit my Thermo Top C into the new (for me) Boxer van that I have waiting in the wings. I would put it back to being an engine block heater as well as a defroster.
I have a diesel air heater in my current van and it works well, but it doesn't warm the engine as well as the cabin which is something I would like.
