I've been in the house for around 14 years now
OK. I was just checking that you were able to compare the performance last winter with earlier years.
As you will realize, last winter was very cold, with temperatures dropping to well below the -1C used when heating systems are designed. This would account for the poor performance last winter. However it does not account for your current problem.
You first raised the problem in August, when the weather was warm. Did you turn the boiler thermostat to Max and the room thermostat to a high temperature, e.g 30C, to make sure the water had time to heat up before the thermostats worked? If you didn't the thermostats could easily have cut in early.
Have you balanced the system? It's essential.