As above, most NSH are very simple. Most problems i encounter are that the people using the heaters don't know how to basically. I always tell people to set the input to about 4-5 and the output to 0. The output controls the flap on top the heater witch is connected to the output kno
b via a by metal strip. If its set anything about 2-3 the flap opens too soon and the heater will run out of heat by the evening. Leaving on 0 keeps the flap closed right till when it needs to open on the evening
Fan assisted heaters don't have a flap on top instead they use (you guest it) a fan to expel hot air from the heater. They have a E7 and 24H Supply. Some also have a 500w-1kw "boost" element on the 24H side.
On most older dimplex/creda heaters if the stat is set too low the heater wont charge up at all. The stat has to be closed to put a charge in. If the room temp is already above the stat temp it wont charge.
As above there is always a overheat cut out this can be a thermal link that melts and requires replacing or a push button type witch you can reset. Common cause being the vents blocked.
Elements can fail tested buy load testing OR resistance test.
Some of the more complex heaters use relays and electronic PCB control gear. These will generally be fan assisted heaters from experience.
Some systems use wall stats. Can be a bit misleading if your not aware of the wall stat straight away. Also seen a few systems with a central control panel at the mains using wall stats and even a out side temp sensor to control the heating.
HTH
Ben