Open Central Heating system with an expansion vessel.

It's a fairly common arrangement, I expect it was done that way by house builders to avoid call backs due to loss of pressure in a fully sealed system. In the event of any small leaks the system will be topped up automatically from the cold water cistern in the attic, and in theory the check valve prevents back flow from the heating system to the roof tank. In reality it's not a great idea for several reasons. It can mask a system leak, and the constant topping up which will lead to corroded radiators. A leaking check valve will allow the heating system to contaminate the cold water cistern, assuming that a separate feed tank is not installed, which is often the case, and a sticking check valve will prevent the system from filling at all as usually there is very little head pressure available to open it.

A fully sealed system filled from the mains water supply via a detachable filling loop and with an appropriately sized expansion vessel is better in almost every respect, other than you need to keep an eye on the system pressure and top it up manually once or twice a year.

Sealed systems are common, by definition they do not have an open feed pipe, expansion is taken care of by the expansion vessel, and if it fails, by the pressure relief valve within or close to the boiler. In your case before filling the heating system check that the bladder in the expansion vessel is pressurised to around 2 bar and is not leaking, if so it should be safe to use but I would consider converting it to a fully sealed system when you can. It would not be a major job. You may find that the pressure drops off more often that it should, in that case you will need to find and fix any leaks.

have to agree pipe is just pipe it just a tube that holds water whether the heating system works or not is down to the competency or not of the person who installed it

You probably have whats known as a semi sealed system where the cold feed has a non return valve so that it becomes a pressurized system every time it heats up, very common around here (50 year old systems) where, when gas became available, some people then changed to gas fired boilers, most then also converted to these semi sealed systems. Even if the valve is closed (with no NRV) then no danger since you have a pressure relief valve on the boiler.

Would suggest you use a swing check valve with a rubber seat installed horizontally, the spring loaded type may cause lack of water in the upstairs rads. Then install a isolating gate valve on the feed&expansion cistern supply to the ballcock and close it after refilling, by doing that you can monitor any leakages, is there a pressure gauge anywhere?.