Could life that with 4 jacks, but you would need to cut slab back to get jack in. Had a thought, there bumper lift lifting attachment available for hi lift jacks, they pretty compact, might wiggle them under.
That slab is history anyway - not a lover of concrete in the garden. I want as much earth as possible. Isn't the slab thick enough to prevent moisture getting up through from below?
These new posts are reply usefully. I've just completed my garage roof in epdm. Was most impressed... think that will be my roofingredients material for the disastrous damp shed. I will be back with wall and insulation questions no doubt.