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How much do you top up with ?
Then it is only one of two things.
1. There is still a leak - but very small.
2. The pressure cap is leaking.
This requires a patient pressure test from your garage.
They hook up a radiator pressure tester to the system and leave it on for a good while.
Since the leak is so slow maybe an hour. - It usually only takes a minute.
It should be done when the engine is warm.
Similarly they can test the pressure cap.
There have been issues with head porosity so the leak may need a chemical treatment t cure the leak - a bottle of holts ïrontite" will do the job admirably - follow the instructions on the bottle.
Get it tested soon.
The expansion tank is the right size.