It does modulate the boiler output on central heating, it can work with outside temperature compensation only, room compensation only, or a combination of the two. My criticism of it is the timings of the hot water seem a bit random. There is a clock schedule for it, but the hot water is always...
It is not to prevent backflow. The two port valve closes flow through the TMV and straight back to the return. With no demand for the underfloor, the valve will get to temperature and open the path to the boiler return.
Oh Dear, I've already put a deposit on one.
Seems to have had plenty of oil changes and I've read about the swirl flaps and rear suspension.
Anything has got to better than a 17 year old Mazda Bongo though...
Sorry to hear of your troubles, but I think that you should have scrapped it at the turbo.
I had an 02 with 184000 miles on it and thankfully someone wrote it off for me.
Go to an auto electrician and get a full diagnostic.