Loads of TT PR activity on this one.

EG:

http://www.mirror.co.uk/money/personal-finance/talktalk-launch-cheapest-mobile-phone-2363295


What they don't make clear in the media coverage is the sim only plans are exclusively for TalkTalk customers.

See: http://sales.talktalk.co.uk/product/mobile/index for plan details etc.

It doesn't undercut rivals by 50% as reported as Virgin Mobile offer one to VM Customers for £5  with only a 30 day term.

http://store.virginmedia.com/virgin-media-mobile/sim-only/pay-monthly-sim.html?buspart=google

And Vectone offer a £5 a month one with no commitment to anybody.

https://www.vectonemobile.co.uk/paymonthly/Pay_monthly.aspx

and cheaper still Shebang offer one with very few minutes for £2.25 a month . https://www.shebang.net/pages/easy_application