Cistern in bathroom briefly filling every 5-10 mins

[Steve111111]

Maybe every 5-10 mins the cistern in the bathroom downstairs makes a filling sound for about 2-3 seconds. I'm thinking maybe something is leaking internally which is lowering the water level and the tank needs to top it back up. TBH I don't really know a whole lot about plumbing. No external leaks that I can see.

A took a photo of the inside of the cistern. Maybe something is obvious from this?

BTW the plumbers that fitted the house put in the cheapest crappest parts (main plumber even told me this himself. Builder that contracted him wanted the cheapest price). So I suspect a part just needs to be replaced. It's about 7 years old.

I can take some more photos to get better close-ups if required.

 

[CountryFan]

Check the back of the WC pan inside where it would usually flush from, if water trickling there then it's likely the flush valve seal has failed.

 

[opps]

Check the back of the WC pan inside where it would usually flush from, if water trickling there then it's likely the flush valve seal has failed.

As above... sometimes the trickle can be so minor it is difficult to see. If you put some loo paper on the interior of the rear of the pan, if it gets wet, you will know that the water is running down.