The tests for the second coat of the clay render are now complete.  The mixes we tried are:

Mix 1
1 clay: 2 sand: 1/2 straw

Mix 2
1 clay: 3 sand: 2/3 straw

Mix 3
1 clay: 4 sand: 1 straw

Mix 4
1 clay: 3 sand: 1 straw

Mix 5
1 clay: 4 sand: 1 straw