Concrete should be okay, but make sure any drainage from the bin goes away from your guests' Guccis. You may also need to make sure that it starts with the right inoculant, and it doesn't stay too wet. On the plus side, you won't have to fight the tree roots when turning!

On the downside - how will the worms get in if you site it on concrete?

Better to have on soil unless, like another GoY member you have a problem with ground elder or another pernicious weed.

We have a compost bin on concrete for yonks. Seems to rot down Ok. Our worms love cracks in concrete.....they have their own way of getting in and out of our bunker. Every so often we top up with those little red worms from Wiggly Wagglers......not very often. Also worms get press ganged from the garden into the compost too. Nice sunny position is handy to help with heat

Good thinking Dorjac... In an 'ideal' world all the bins would be on soil but a compost bin on concrete is better than no compost bin at all.

I confess that I forgot about the little wigglers! Subconciously, I guess that I was thinking of them as part of the "inoculant". But, if I meant it, I really should have said it. Oh, well.