The house does sit low compared to the sheds so water has to rise after the house to trough a bit . The pipe coming in house is 25mm so there must be a reducer outside under concrete
OK, I am suspecting that maybe you have had a reduction in water level, thus you see the water flow when you lower the pipe 2 feet below trough level, then stops when you raise it. This could also be your issue with the boiler pressure, as 0.5 bar is not enough pressure for some boiler to operate effectively.
The way around this is to fit a booster pump to the main incoming feed (before the T off for the barns) - so do you have an area that one could be fitted and powered up?