FBH is a fuel burning heater, bit like a micro central heating boiler, used to defrost the screen, pre heat cabin, and lets the temp needle go past 7 o clock.
If it has just stopped working, you will notice a HUGE difference. But I do not see how your temp gauge goes up to half way without one. that combined with low cabin heater temp, says low coolant to me first off. Have had several that would not go up to half way in summer. Look at the N/S wheelarch liner just ahead of the front wheel there is a hole, if there is a mini exhaust pipe there facing downhill, you have one.What happens when you turn A/C on.