The boiler efficiency argument here is marginal and is at the expense
of performance. You are sacrificing the stored capacity for a
presumed improvement in efficiency.

However, it could well produce good results, provided that the thermostats
are both near the bottom. It allows a much more precisely defined hysteresis
to be used. I'd probably use that at the 95% full (to turn the boiler off)
and 75% full (to turn the boiler off).

Should a flow switch kick option be required, this could be achieved without
a 3rd overheat thermostat, by kicking the latching relay into the on
position if the 75% full is happy with that. A further advantage of this
system over a separate overheat stat is that the boiler then fully recovers
the system when hot water has been used, meaning the tank is usually 95%
full. The only way for it to get less than that is through natural heat
loss. Simply blipping a tap on would ensure 95% capacity a few minutes
later.

As I have a heat bank, I already have a flow switch in circuit, so all I'd
need is a few relays and another thermostat to implement a full wide
hysteresis system.

I may even implement such a system, as I think I would gain much efficiency
using it. On my current system, the boiler is modulating down and keeping
the primary circuit running for very extended periods. Unfortunately, the
automatic bypass valve (which is on a long run of primary pipe that wouldn't
otherwise be used for DHW only) seems to be opening (despite the pump graph
and setting saying otherwise), wasting lots of energy, particularly as some
of the run is unlagged (buried in plaster).

Christian.