Lower Hose & Temp Guage - Cooling System

[Gibbo18]

Hi All

On the back of making my first post last week and receiving a number of replies (thank you so much for taking the time to do that), on a related note can I ask for any feedback on why my Z4 2.0i can drive for 80+ miles with the lower hose barely reaching lukewarm and the temp gauge only occasionally hitting the middle point (50%) only to then drop off to around 35-40%.

My previous post advised that the yellow empty rad light has come on periodically with the rad subsequently found to be either empty to have lost a lot of coolant/water. In endeavouring to resolve this ongoing issue the car has been fitted with a new rad, new thermostat (x2), new rad cap (x2), new water pump and new header/expansion tank. In addition to that the car has undergone 2 x sniffer tests to square off the head gasket angle. Apparently that's a 'full set' when it comes to the cooling system, hence why people that know a lot more than I do about cars are completely puzzled. Any help gratefully appreciated.

[G600]

New o ring on the coolant temperature sensor? There are also pipes running to the bulkhead and the heater matrix on the other side to check