Yeah this is the type of thing I am going to end up doing. I do have some 200 plastic packers and 50 wedges plus some old upvc strops.I built over some old paving, I just stapled dpc to the joist and sat it on the old paving, I levelled it up and packed cement into the gaps at 6" ish spacing or sat it on bricks or bits of broken slab bedded into cement depending on the gap size.
