cooling issue or airlock

hi all i wonder if someone could help me in this long stressfull time of
cooling problems,basiclly ive got no heat coming through vents,losing
abit of coolant,engine never overheats,top hose is hot bottom is
cold,changed thermostat 5 times and doesnt make any differance at
all,ive tried bleeding it from the heater matrix but to no avail
although when i pulled the pipe this morn there was no water coming out
at all plus none returning to the expansion tank,the
top hose isnt rock hard when hot and no usual signs of headgasket so i
dont think its that,im wondering weather the rad is blocked,another
faulty thermostat or water pump has packed up,ive looked for leaks and
everything plus after a 50 mile drive the water in the expansion tank
was only very slighty warm and there was no pressure at all when
releasing the cap,this really is driving me up the wall any help will be
very much appreciated thanks