Your system sounds quite old and it is open vented. You are also getting gas or air in the radiators. Very likely it has sediment and sludge inside, so it is always worth giving it a clean. Taking off the radiators and squirting them through is a good way to remove a lot of it. To loosen what is in the pipes, you can use a chemical cleaner, such as Sentinel X400 which is very mild, you add it to the F&E and let it circulate for 4 weeks before draining. If you are passing a plumbers merchant or DIY shed today, buy a bottle (it will only cost you about £15).
If you are planning to take the rads off and squirt then through within the next week, do that first. If not, add the chemical and it will loosen the sludge before you squirt it out.
Go into the loft and look at the F&E. It will probably have mud in it. Tie up the ball valve, bale out the mud and sponge it clean, then draw out a bucketful of water from a drain cock downstairs.
Adjust the ball float so there is only a couple of inches depth of water in the F&E above the outlet near the bottom.
Then add the chemical and stir it in as you untie the ball cock. This will draw the chemical down.
Observe the F&E when the pump is running and see if water is coming out of the vent pipe which is curved over the top of the F&E. If so, turn the pump speed down and see if this cures it.
As the X400 starts to work you will find the water in the rads goes jet black, this is with loosened sediment that the chemical is washing round.
It sounds like you have a Gravity Feed to the cylinder. This is inefficient and there are a couple of ways of improving it, which we will discuss after seeing the photos.
Measure the pipes between the boiler and the cylinder (22mm or 28mm) and buy some Climaflex or similar stiff foam pipe lagging. A lot of heat is wasted from these pipes. DIY sheds tend not to carry the 28mm size. Get the thickest that you can fit.
Look for any signs of leaks.
