Knit toe-up with afterthought heel.
This pattern is no longer available from Mencap, as the campaign ended in December 2010.
Spiralling socks is no longer available online from any of the listed sources. You may be interested in the Spiralling Socks eBook, which contains two related top-down sock designs.
This pattern is written to be worked on DPNs. If you wish to work using circular needles, then rather than knitting over a set number of needles of stitches, ~work each yarn to 3-8 sts before the following yarn is joined in. Then slip those 3-8 sts from left to right needle (so that the last st on the right needle is joined to the new ball of yarn). Repeat from ~ to make the main body of the sock pattern.