The are a few things to consider first. What height is the concrete in relation to your airbricks as you must not block them in any way and this will determine the size of bearer. Is the concrete even and what fall does it have, as you will need to block up the bearers. I would prefer to run thru a few rows of bricks (1500mm centres for 4x2 and 2500 for 6x2) and rest the bearers on these on DPC. A deck can really smarted up a tired old area, but please plan your job first - I ripped out and renewed two decks last year after enthusiastic DIYers had made a complete b*lls of them - one could have doubled up as a trampoline and the other was made with untreated 10mm planks which had given way after a couple of summers.