I'm on the same package, having had Ntlworld/ Virgin work reasonably reliably but slowly for a couple of years. I made the initial mistake of following Sky's advice that for an extra £5 per month I could get up to 8 Mbps. Turned out I couldn't connect at all for longer than a few seconds for about three weeks and exhausted all of their so-called technical help lines with no improvement. They sent me a new modem, which stayed connected a bit longer but got very hot.
I eventually got it working after a fashion by myself (I think the quality of the phone line was a factor, BT were working in our village). The speed was painfully slow at first but it did improve and now works at about 1.5 Mbps. It was obvious early on that I was never going to get more than 2 Mbps so I complained and got the contract changed back to the cheapest option.
If in doubt, ditch it and try someone else's service if the hassle of changing doesn't put you off.