Hi , I apologise if I seemed to be preaching .
I only said that because I had exactly the same symptoms on a previous car and was also told that it was the head ..
The reason i refused to accept that it was , was that the lack of warm air coming from the vents proved that the water couldnt be circulating.
Therefore it had to be the pump .. as a thermostat stuck closed would still allow water into the heater matrix and i would have had warm air from the vents.
Insisting and digging my heels in , saved me a very costly head job.
A word on block testers ... I have one .. they dont always work.
I have seen engines with blown gaskets test clear , and also engines which fail the test to be just a result of contaminated coolant .
And yes I am the fool that pulled the head on a perfectly good 4x4 as a result of a false positive test.
Use your noggin, think about it and ask for a satisfactory explanation as to why there is no warm air from the heater vents if the water pump is working and circulating the coolant as it should be.
Every time you start a cold engine the thermostat is closed and still the heater defrosts the windows long before the rad gets hot.
A word of caution though , do NOT leave that engine run very long unattended or you WILL need both a new waterpump AND a head gasket.
Hope it works out well for you ..good luck