My car (TR7 engine, so close enough) had a weep along the inlet side of the head/block join. Slow, but there.
Like you, no other symptoms. So it could be gasket. Assuming you have had the inlet manifold off and checked/new gasket etc etc. and obviously you are certain not the water pump.
As to the header, height won't matter. The takeoff at the top, just under the car, is after the sealing washer so unless the system over-pressurises and forces teh cap seal up/open, no problem. And when cold, the water does not flow out because that would create a vacuum in the engine, so the 14psi of air pressure we have at sea level keeps that from happening. Don't worry about it! The tank should work fine as long as the cap seals ok.
It could be the pump but the water on the block appears to be coming from above it. So it is either the head gasket or the inlet gasket.
The rate of loss does seem to be reducing, so I am hoping the leak stop may have cured it.
Irritating because apart from that there isn't a leak on the car, very un-Triumph like.