In addition to T&M, Fothergills are offering earlies or "Christmas potatoes", but if you're not placing an order for other stuff the cost (inc P&P) will work out at around £8-9 for 10 spuds. :( I've found more expensive retailers... ::)
I planted some rather sad and shrivelled International Kidney seed potatoes (left over from spring) into a tub last weekend; they are already up. If they produce a reasonable crop, and have an acceptable flavour, I'll simply increase my 2010 order of earlies and retain some for autumn planting.