Never before in any car have I owned have I had to top up coolant level in between annual service . It's happened again the warning a few months before service due and the level is below the minimum line. Should I query this with my service centre or just buy some coolant and top up myself?

Octavia Rs 2021 49000km

At annual Services it often gets TOPPED up to MAX and by the time you are home it might be as it was at the Level between MAX and MIN. 

Correct, as there is no FILL TO THIS LINE, line.

 

You are loosing Coolant so need to find out why.

Are you running the correct strength of Anti Freeze / Anti Corrosion / Summer Coolant,  AKA as Anti Freeze? 

 

What have you a TSI RS?