Do you need to do something softwarewise to get it to stay at 80C?

No. The software is currently set for a maximum of 88C. Unfortunately, the
little peg in the knob physically prevents you turning the thermostat that
high. The peg is useless. They would have been better to have just had the
default software setting at 75C which would have just ignored any thermostat
knob setting above that value. I wonder how many Greenstars there are round
the country with chewed up yellow pointer knobs...

Can you have it at 80C for DHW and modulation on CH?

No. They had an internal diverter valve option with separate temperatures
for both sides, but the hot water flow is limited to 75C. Of course, it was
only later that it occured to me that I could have run the radiators on the
hot water circuit and the cylinder off the radiator circuit to bypass this.
OTOH, this may have defeated some internal algorithms.

Christian.