The toilet cistern will not fill up. The water is going into the cistern OK .But

After the cistern only gets a small amount in it, water starts going down the

Toilet pan?

Any ideas?

Yes, You may need adjustments in the tank. Look at the flapper, If it has wavey edges on the flat surface, replace it. Now check the flapper linkage
With the flapper seated the linkage wants to have 1/4" of play. Too much play and the flapper doesn't get pulled back enough giving you a short flush. Too tight and it lets water seep past the flapper. I adjust the chain by the link and if it needs fine tuning, I bend the flush lever rod a bit untill I have the desired play in the linkage. One more thing about a flapper. you will see where the old flapper hooks onto the base of the white overflow tube. On a older type with no hooks the flapper has a ring that slips down over the overflow tube to the seat. If your tank has hooks, take a sharp knife or scissors and cut the neoprene ring off on the marks provided and hook the flapper on the hooks. Leaving the ring on will interfere with the flush. The water level in your tank should be 3/8 to 1/2" below the top of the overflow tube when the tank's filled. Hope this helps, Tom

There is no chain involved in the cistern. The flush is connected by a lever

Which when flushed presses down on a tube in a tower type housing. At

The bottom of the housing a cone lifts, which is at the bottom of the cistern

And the water flushes down the pan. This housing contains a number of

Springs and rubber seals. Sorry if my earlier explanation wasn't full enough.

The fittings in the cistern are about 4-5 yrs old.

Can you take a picture of the flush valve or find a brand name on it? Regards, Tom