Andrea, you have a challenge but a good one.
Yes build on top of the clay, which in time will open up.
Check the horse manure for aminopyralid (herbicide) by sowing some peas & broad beans, on a warm windowsill for 3 weeks, if new leaves are healthy you should be ok.
Six to eight inches (15-20cm) for beds, say 6in of the horse manure then a top layer of green waste/mushroom compost, or compost from stores in sacks (sometimes sold cheap if out of date).
Use any less-decomposed materials for paths, including strawy manure say 2-3in.
Starting out it’s easier to have sided beds to contain the compost, also for community workers to see where to work.
Happy growing.
