The new act comes into force on 1st October 2015 and protects consumers for purchases from that date. It replaces three other pieces of legislation: The Sale of Goods Act, Unfair Terms in Consumer Contracts Regulations and the Supply of Goods and Services Act. If your contract started before 1 Oct 2015 then those pieces of legislation still apply to you.
The rule regarding proof of fault remains unchanged - any fault identified within six months is assumed to be "inherent" (i.e. manufactured in). It is Virgin's responsibility to prove that the fault has developed as a result of misuse. After that, and up to six years from purchase, the consumer has to prove that it's an inherent fault. With regard to Virgin Mobile, there are four important changes in the new legislation:
30 day rejection You've always been able to reject faulty goods, but it was within a "reasonable" time. Now, there is a defined timeframe. You can reject your Virgin Mobile contract (see point 3) if the handset has a fault or if the coverage turns out to be poor despite what the coverage checker tells you. Note this isn't the same as the 14 day "cooling off" period provided by the Distance Selling Regulations. This allows you to cancel your contract for other reasons, such as not liking the colour of the phone or it coming with an A/C adaptor whose shape means it can't be used in the socket you want to use.
One chance to repair After one month and before six months, you have to give Virgin the option to repair or replace - same as before - but now, if it comes back and the fault is still present, or the replacement has the same fault, you can demand a full refund.
The handset and tariff are one product The Freestyle contracts might be an attempt to get out of this, but I don't think it would pass scrutiny in court. Your phone contract is for your handset and your connection - the new legislation pairs them up, so if you cancel because of a faulty handset, you cancel the service as well.
Promises made during the sales call which aren't delivered count as a fault The new act is an attempt to simplify problems with digital goods and services. One outcome of this is that if you are promised a service (such as 1GB data) and you don't get it (you get 500MB) then you can reject. I always recommend that you record calls to any organisation where you are making commitments to pay money based solely on your word against theirs.
I'm still reading the legislation so there may be updates to this thread as I find new information.