The pipe is now insulated.... and we are getting reasonable daily results from the Kingspan Tubes - 5-6-7 kWh/day for these sunny days of the end of May.
If we could double the area of tubes, it would not automatically double the capture (because it is subject to other factors such as pump speed, delta T and more), but it would make it more acceptable. And if the tubes faced south, it would be even more effective. So perhaps 4sqm facing south could get about the kWh as the Sunbox - 14-18 kWh/day on days like this. Now that I have sorted out the plumbing requirements, I feel ready to try it for someone.