Low head of water can be overcome by raising the loft tank, ie constructing a staging to hold the tank higher in the roof area. I have done similar to increase the flow to showers which need a minimum of 1m. above the shower head to the base of the loft tank to give a reasonable flow.
As to the hot water air-lock problems and hot water flow - are you in a hard water area? as I have had to replace a few top cylinder outlet connectors, could be either a 28/28 or a 28/22 male iron/compression depending on the outlet pipe diameter, as they have furred up with limescale and reduced the water flow out of the cylinder. The water flow to taps is reduced and the water in the expansion/vent pipe is sucked down, plus the air above it into the tap flow pipework. This I think is what is causing the low hot water flow and air locks you have descibed. Remove top cylinder outlet joint and check/replace if clogged with limescale. Have had instances of previous joints internal diameter reduced from 28mm to less than 15mm due to limescale. In one instance to less than 4mm, causing water flow from taps to stop after a few minutes, even though taps were still open.
Lee
As to the hot water air-lock problems and hot water flow - are you in a hard water area? as I have had to replace a few top cylinder outlet connectors, could be either a 28/28 or a 28/22 male iron/compression depending on the outlet pipe diameter, as they have furred up with limescale and reduced the water flow out of the cylinder. The water flow to taps is reduced and the water in the expansion/vent pipe is sucked down, plus the air above it into the tap flow pipework. This I think is what is causing the low hot water flow and air locks you have descibed. Remove top cylinder outlet joint and check/replace if clogged with limescale. Have had instances of previous joints internal diameter reduced from 28mm to less than 15mm due to limescale. In one instance to less than 4mm, causing water flow from taps to stop after a few minutes, even though taps were still open.
Lee