Hi Derbex, about 3 months ago someone posted excellent pix of carrots that doubled back in the loo roll when they hit clay, like you describe. Eeew! :P
I'm going to drill holes this year and plant parsnips in the top of a cone of sand/compost, as advocated by MikeB and Curry, once I have built a patent Parsnip Planter.
I also have scorzonera started in loo rolls - to encourage long roots like moonbells say - and will plant these in builders bags of compost (spent hops, coffee grounds with added FB&B). Have only had this plot 11 months so still experimenting :D