You really shouldn’t be working at anything to do with your Megaflo, except doing the air gap.
The thermostats for immersion and for operating motorised valve (heat to coil from Rayburn) are part of the critical safety components of the unit.
Only a unvented G3 qualified person is supposed to work on those. Excess heat is highly dangerous on unvented units.
Check that no water is ever dripping into tundish, especially when cylinder is up to full heat, as this will lose you hot water and can be due to lack of air gap, or other faults.
Note that all unvented units need serviced once a year for safety and warranty, which is usually 25 years