I can only talk about my experiances i had sky broadband i live in s5 I got around 4 gig pretty much all the time, However i swaped to virgin around a year ago on there 30 meg package i get between 31 and 32 gig all the time.
The problem i had started around 2 months ago and got worse like said previously i rung virgin, Got a new router and it solved the problem (touch wood).