Coolant Header Tank

I am loosing small amounts of coolant, I gather this is relatively normal. I may have lost something like 3-4 litres in 6 months.
Where should I be looking for issues as I don’t see anything leaking?
The header tank has many small cracks around the cap, is this an issue that means I need to replace it?

Hi, yes I would replace that, along with the cap, if it's got those small cracks. If it's a TD4 then it shouldn't really lose any coolant, I know mine hasn't & it's close to 180,000 now, apart from when the radiator went & the EGR pipe got a pin hole in it,see pic .