I've just bought a 2020 Mk3 estate. I want to add a spacesaver tyre. Can I buy a new foam insert to hold it in place to replace the current foam which is a part circle that holds some kit including an inflator pump but also two storage box sections

Cheapest option is to buy the spavesaver, foam insert and tool kit on eBay or the like from an auto recycled.

Can you use the extra foam box section with a spacesaver? Could you use your existing? You will likely be able to buy anything that was optioned with the space saver from skoda (expensive) or an auto recycler. Google search "octavia mk3 space saver kit", and "octavia mk3 spare wheel foam storage".