The additional benefit of knowing this information to a
control system is marginal at best.

Actually there is some value.

Well, maybe some, but it isn't even slightly critical. A large amount of
hysterysis is actually desirable, so that the HWC can be heated in one
(relatively) long burn.

What your sensor isn't telling you is the relationship between the measured
temperature and the amount of energy stored. I strongly suspect that there
isn't a simple direct mapping between the two, as it depends on many
factors, such as the recent history of water takeoff and heating, leading to
different levels of stratification. A simple thermostat will turn the boiler
on when the stored energy is below a reasonable value, so it perfectly
adequete in terms of the function of the system. I doubt that an analogue
control system with a single analogue sensor could substantially increase
the seasonal efficiency of the system.

Have you done any calculations on your system to ascertain whether this is
true or false, though?

the boiler wouldn't come on until the cylinder surface temperature was
at just over 50, then on the way up, the boiler wouldn't go off until
around 67 or 68 degrees.

But that sounds quite good. It waits a minute or two for the bottom of the
cylinder to get properly cold before turning on and doesn't turn off until
the HWC is well and truely warm. It will only take maybe 10% of capacity for
the bottom to get cold enough to turn on the stat. Where this is useful is
overnight or other times without any hot water drawoff, as it allows the HWC
to cool down a bit, rather than trying to maintain a precise temperature,
which results in excessive burner firing and unnecessary primary circuit
losses.

Christian.