Tried a sheet of Polycro as a groundsheet protector on my last trip. I cut it 10cm smaller than the groundsheet of the tent
There was a light breeze and I had the devils own job trying to get it to stay on the grass whilst I pitched the tent above it.
I've thought about very small pegs but worried about damaging the ground sheet of the tent
Am I missing a trick here ? or do I need to make friends with an octopus to hold it down for me

Try tying it to the peg out points on the groundsheet. You should be able to bunch the polycro up enough to form something you can fasten a thin cord to.

The second part of this video https://www.youtube.com/watch?v=VZ3IgELMUUk shows how to quickly tie a line to the corner of a sheet.

Can you pitch the tent as normal and then slide it under? Should be fairly easy with no weight in the tent.
Probably easier if you fold in such a way that partially unfolding exposes the full width. Unroll then pull through a bit at a time.

My tent footprint has adjustable straps same position as the tent inner footprint.
I can use the same peg or separate.
Have a word with an upholsterer or sailmaker they can make you up what you want.