I am sure that the Pipex price looks good and am tempted however I am aware that they are charging what appears to be less than cost.
Nope. Wholesale cost to the ISP is around £15/pm (see
http://www.adslguide.org.uk) - Pipex cost £20/pm + VAT
You may have got confused with the activation charge, which is £25 I think (was more when I joined), or you may not...