All 13 plants eventually withered and died. Once they'd died I cleaned the soil off and found that none of them had put down roots, that the shooting tip of a Rubus plant has the power for upward growth but not downward growth.
This is good. I know something about brambles now that I didn't know before. As they say: failure is always an option.
Moving forward then. For the next batch of experiments (2014 at the earliest) I'll need to graft a shoot onto a root before burying. Grafting on that sort of small scale is incredibly fiddly and up to now I've only ever made one plant graft in my life. For now then, I suppose I had better get some practice in.