Having a nightmare with them at present, I had moved onto O2 to take advantage of the Volt upgrade.
Was previously paying £37 for M500 standalone (3 months to run on that contract), following my Volt "upgrade" I am now paying £38 for M500, when I rang them to point out that this wasn't quite doubling my package for the same price I got complete disinterest, they just said the 1 gig was expensive so they weren't going to do that, I must have missed that in the Volt marketing spiel.
Also if I now take advantage of the O2 cooling off period as I haven't received the benefits of the Volt upgrade (I only really wanted the speed increase, don't need double the O2 data), and lose Volt they will put it up to £56 a month, they rarely are a shameless company.
Trying to go through complaints at present but based on past experience with their customer service I won't be holding my breath