0800 052 2000 Should be open today..

*(Open till 6 I think)
But, They usually take a couple of hours to upgrade your speed anyway, I found myself waiting till the following day before it came through to my STB .. Might just be worthwhile to wait until tommorow, and upgrade through your local CS.
And, Welcome to
nthellworld
Edit : 0845 650 0121 is Faults I believe, Not freephone though.