Lay 3 x 3 (75 mm x 75mm) timbers, well preserved on bricks and lay the
shed
on that. You can do the final levelling up with bits of damp proof felt or
bits of slates.
I used 3 old railway sleepers coated in used engine oil. Made sure they
were level and as I built the shed i screwed the base down to each of the
sleepers. Never a problem


Steve