Who is your ISP? I am with NTL and I started off at 600 which was then upped to 750 and now to 1mb, (in fact it should be 2mb but as my set top box is only capable of an increase up to 1mb that is what I have, unless I ring them and get a new box installed).
Most of the BB suppliers are in the process of upping the speed (in case they lose you to another). Perhaps it's not your turn yet, after all it's only just "the end of March".
Give it some more time.