I want to erect a new shed (3.5m x 3m) The area it will go is currently covered in large paving slabs and whilst these are quite solid and have been down for around 10 years they have become uneven over the years.
If I frame the area could I put the concrete directly over the slabs to even them up (prehaps with a damp proof membrane, builders left some when they build conservatory). If I can do this how thick should the concrete be and what grade?
I wouldn't really say that was a good option. It would be far more stable if you broke up the slabs and used them as extra compacted hardcore under the concrete. In which case 100mm of concrete would be more than sufficient for a mere shed.
The uneven slabs under the concrete (even 100mm thick + concrete) could settle and move even more which could cause cracks to form.