How far do I need to lay paving slabs from damp course?

If the existing ground is the same level as the damp course you are already in danger of invasion by damp. The general rule is as stated elsewhere that the ground should be 150mm (6") below the damp proof course. So you should do something about the existing condition before you lay any slabs. You could consider digging a small trench say 6" wide by 6" deep along the length where the damp proof course is affected and then lay the slabs up to the edge of the 'trench'.