UPDATE
According to the Disco_3_FBH_Workshop_manual_thermo_top_V.pdf the FBH runs full out until the temperature reaches 80 C, then runs at half power until it gets to 90 C. It won't switch back on until the temp falls again to 75 C. This is fairly close for a thermostat of 88. Of course the manual says the temperature set points can be altered by the customer-specifications.
My ?guess? is you have two problems. 1) the 5 Celsius switch is closing at way above 5 Celsius, and 2) Your thermostat is on the way out, and only regulating to 85 C, which is enough to confuse the FBH. These are both common faults, apparently.
If it were my landy I would replace the switch and the thermostat, but I might be tempted to try to alter the customer specific setpoints from 80,90,75 to 75, 85, 60..if I had a hawkeye