so many variables

[charles]

That is a good point about the sacks lying wet which certainly could be the problem if it lasted a while. Comfrey with leafmould sounds a good idea, few weed seeds. Some people advocate cooking the compost to 60C in a low oven to kill weed seeds but I feel it kills too much else, so I have no answer except lots of weeding. Maybe sow into bought compost then prick into your own, plants are then two leaves ahaead of the weeds.