Ah ok. Makes sense. Just tried it out and don't think that's it. It was hot from before, ran for 10 minutes and didnt cut out. Return pipe scorching hot.
The expansion vessel needs recharging...pressure too high when rads hot.
The heat is going into the rads..it can't disappear. Give it time and the boiler should modulate down or shutdown on overheat.
It's always a good idea to set the output to match the heat load...set the spindle on the pcb to an appropriate setting...minimum for flats and 1/2 way for larger homes...unlikely to ever need it at max given it's a combi.
You're better off junking it. It was a disaster the day it was launched around 1996 and most were ripped out within 5 years....soo many design faults.