Hi Grizza
I guess he turned the body around for access to the wiring and speed control then; as if you hold a new pump in the position of yours, with the flex inlet on the top left, which would be the bottom, if the pump was upright, the arrow points to the right, which in turn, would be the top of the pump.
He could maybe split the cold feed to the F&E tank and blow it down the cold feed inside the tank and also the vent. I have done that before and cleared either an airlock or a lump of black iron oxide.
Simply draining and refilling a system will only clear out what is in suspension in the water and not what is trapped in the smallest of places or has compounded itself after years of flow direction.
If you squeeze a wet tea bag, all the leaves will compact, run water through it, some will break away and float, but the lump will remain a lump. (Poor analogy I know)
Id be thinking of a powerflush, but if you do that, make sure the vent and feed are connected together and your 2 port valves are manually open.
Cheers
Mark