1.9tdi 130 mystery coolant loss

[fz911]

hi guys, I have a strange problem, ive been losing coolant for a while so ive replaced the radiator (It had a leak)
afterwards I noticed it was still losing coolant (1 litre every few days) ive done a coolant pressure test and its all ok, I noticed after a 25 mile drive that the bottom hose was still cold, I assumed the thermostat was at fault so ive replaced that this morning, the old thermostat was seized, but now the bottom hose is still cold after running for ages outside the house and then a quick drive up a junction of the motorway and back.
my temp gauge shows 90 degrees or thereabouts on the dash.
any ideas why this bottom hose would not be running hot with a new thermostat in it (78degrees opening?)
cheers