Aunt Sally, I'll try the golf balls (will it really work) and yes the shells are thin. Rich, I feed them on layers pellets and some raw veg scraps. What happens if too mcuh treats(not that i think they do get) but for future reference. I put in oyster shell grit stuff as well but they don't seem to touch it, the run is bare earth now so probably getting grit from that any wasy.
If you feed 'em too many treats then they can get fat which can cause problems for them laying etc. but raw veg scraps are not treats so don't worry about them.
You could try sprinkling a little ground oyster shell in their food to fool them into eating a little more. You will be amazed how quick they will strip out any usable grit on bare earth so it's good to supply extra, I've sometimes thrown a bit on the floor of the run they pick it up as they need it.
BTW just in case you don't know oyster shell is different to grit but both need to be supplied. Some times they also benefit from a drop of cod liver oil say once a week to help boost their vitamin D levels as this aids calcium absorption try giving a drop on a small piece of bread they soon scoff it.
And yes Aunty's golf balls work