newbie with coolant issue!I am

Hi guys,
I am a newbie on here and a new freelander owner with an issue and wonder if anyone can help with guidance. I have a 54 plate TD4
Here are a list of points all based around my coolant problems.

1. My expansion tank is filling with froth, this is being fed in via the little pipe at the top and not from the bottom pipe.
2. The bottom heater matrix pipe is warm and the other is cool. I removed the air bleed screw on the top hose and at idle the coolant only dribbled out. The coolant is mirky not bright red.
3. The heating inside the car is mild and gets warmer as the revs rise and cools again when it goes back to idle
4. The Temp gauge in the car never gets above 1/4 hot

Do you have any thoughts, could it be an air lock or do you think it's the dreaded head gasket?