Hi Tom,
Just did some research on John Innes compost and there seems to be a recipe called JOHN INNES CUTTING COMPOST which is :
1 Loam
2 Peat
1 Sand
No added fertiliser
This seems like a good natural substrate with no added minerals or lime and the peat should keep ph down as it decomposes into humics.
Any opinion? I reckon about 1.5 to 2 inches of this with an inch of sand.