Megaflo with small extra kettle - pressure loss. Why?

I have a megaflo hot water and heating system in the house and where the tank is, there is a small red tank, about the size of a football, and this has a preaaure gauge. It used to read about 2 bar, but is now on about 0.3bar. The green area is 1.6 to 3 bar so i am worried. What does this small tank do, what is the gauge showing and why has it dropped to such a low level. Heating and hot water still work although the rediators do tend to get a lot of air in them.