My Mum's Halstead boiler is showing fault code C the code is constant not flashing, hot water fine but no CH any suggestions ? I understand this is for the benefit of a service engineer but I need to get out of service mode and return to normal use ! any help would be great

Kevan, October 2013

SERVICE MODE:
The appliance enters the SERVICE mode by turning the control
knob to ‘SERVICE’ (full clockwise). The LED displays a flashing
‘C’. In this mode the appliance runs at the minimum CH output.
This mode allows the gas valve offset and CO2 emissions to be
measured. To increase the output to maximum turn the control
knob back on and return to Service within two seconds.

A1, October 2013