I set up a raised bed this year, one that takes around 1000 liters to fill. So on the bottom half I mixed dirt and uncomposted leaf mulch. Then filled the upper half with good soil. It is doing really well, and in the fall I can then bring the bottom half up to mix with the top half.

I have done that in the past Sheila. They were placed upside down, largest first, and definitely reduced the amount of compost needed to fill the pot. I once used polystyrene chunks - never again though as Woodlice decided to live within them!

Thank you both . . . I shall go ahead with the plan :)

I would also place a piece of plastic window screen, or counted cross stitch screen over the pots before putting in the compost. The window screen has the advantage of keeping critters out of the compost.

Yes - thanks Tug. My OH has suggested exactly that, and we have found a piece of plastic to use.