Potterton prima 50f boiler low pressure

[SuperJonnyC]

We've got a Potterton boiler with a hot water tank upstairs in an airing cupboard.
Every year we close the valves on our radiators because they heat up even with the heating turned off.
We've opened up the valves for the winter and tried bleeding the radiators but there's a lot of air in the system that's not being replaced with water. We don't have a cold water tank in the loft so I'm not sure how we get more water into the system. This has also caused the boiler to fire up for a couple of minutes but then there's a few knocks inside and it goes out. Any help would be appreciated.

 

[terryplumb]

If you don't have a small tank that feeds water into the system ,then you must have a filling loop.pics of pipework around boiler and hot water cylinder may help to pin point it.

 

[SuperJonnyC]

Here is the pipework around the hot water tank