The engineer had installed a Honeywell T6360B room thermostat which is a basic and simple-to-use room thermostat and in my opinion a good room thermostat. According to the installation instructions you should always connect a neutral wire to the room thermostat on systems less than 6 amps, which is not always done. Many installations have a cable to terminals 1 (supply) and 3 (load) and no neutral connected to terminal 2. In this scenario the wires work as a switch either way around. With the neutral wire connected to terminal 2 it works as an anticipator.
The wires in this case were the wrong way around, so the supply was in 3, the load in 1, and neutral in 2. Had the neutral not been connected the thermostat would have worked correctly (although installed incorrectly). Switching the wires around in the thermostat so that they were in the correct terminals solved this problem.
The works described in this blog are a record of work undertaken and should not be taken as guaranteed to solve your problem.