I got upgraded on March 07th to 2MB and then they charged my for:
1 Month 750K £24.99
1 Month 2MB Service (Month In Advance)£24.99
1/2 For 7th March to 26th March £16.99
And to top the lot i'm now arguing with them as it seems as I've just started a new 1 year contract!!! not just an upgrade!!
Apart from that the extra speed is great call them. Just check your bill.