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They're already sitting quite flat and there's quite a lot of rubble I took out from under the hearth that I can pack under it.
I could probably do with some of those little plastic wedge things to pack them in as well.
tbc-
I've got a box of those, not that it's any help to you right now.
You want something solid underneath really because as people walk around the boards will flex slightly and I guess this will over time have a crushing or compacting effect.
I'm guessing you have maybe up to an inch of space below the boards? If you've got something solid like a roof tile you could bed it into some mortar and rest the boards on that but it's probably overkill and it's still going to be quite hard to get it snug against the boards when you lay them on top
rhowe
Think you probably did about the best you could given the circumstances. Rather than underlay it would maybe be better to chock the boards up on wooden blocks but getting them to sit flat on the slab will be tricky.
When I did it on our ground floor the joists were the other way around so I just took the whole thing out and put a new joist in across the hole where the hearth was. It took me some time and created a vast amount of rubble. Then I did it to the other hearth too. Feels like a lifetime ago.