What make of oil filter do skoda use as standard, Bosch, Fram, Mahl, VAG or are they specifically marked Skoda.

Don't know, but any of those you mentioned will be fine, as well as Mann.

Yup, typically Mahle rebranded to VW Group item at initial engine build and typically only Mann rebranded to VW Group item for service parts.

I tend to look for VW Group at sensible prices and also for current Mann prices, and buy which ever is cheapest at the time.