Advice received from the Selfsufficientish forum suggested that there was a danger of the conditions at the bottom of the pots turning anaerobic (ie. airless) because of being constantly wet and fermenting to produce toxins that might harm the plants. I’ve gone round a couple of them with a skewer to make airholes and will see how those compare with the unholed design.
However, I think a bigger threat to the well being of the seedlings might be the low temperature. It got down to below freezing inside the polytunnel overnight and I could feel that the earth inside the pots was lightly frozen. I think I need to begin working on further smaller shells inside the polytunnel, each of which will add an additional layer of insulation.