Hi..i very recently upgraded from 20Mb to 50Mb about a month ago. I got through to india and a nice lady assured me all it would cost me was an extra 20 pound one off fee on top of the 37 pounds i was already paying..after that she said it would be 38 pound a month. Anyway installation was done on time and with no problems and i was very happy..until i got my bill a few weeks later of 93 pounds..an extra 36 pounds..so i would be a little bit wary what they tell you.
Anyway..i just paid the extra as so far the 50 MB has been ultra reliable and very fast..we are really happy with it