Help! I've received a letter accusing me of illegal file sharing!

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Background
Some of our customers have received letters from law firms such as ACS:Law and Davenport Lyons accusing them of sharing their client’s copyrighted material over the Internet. In these letters, customers are asked to pay a disclosed sum of money as costs and damages.
Whilst we don’t condone piracy, it’s important to note that these accusations are those of the copyright holders represented by these law firms, and are not necessarily the views maintained by Plusnet.
The following information is aimed at helping customers who have received one of these copyright infringement notices.
We are not legal representatives and if you need any legal advice then we recommend you contact a solicitor or speak to the Citizens Advice Bureau.
Why me?
The copyright holders and their legal firms employ the help of companies such as Digiprotect and Logistep who use bespoke monitoring software to identify  ‘IP addresses’ being used by people connecting to file sharing networks. They specifically target those uploading content belonging to the copyright holder they’re working on behalf of.
What’s an ‘IP Address’?
Each time someone connects to the Internet they are assigned a unique identifier known as an ‘IP address’. This IP address is typically a series of numbers separated by dots. Using this IP address, we can identify the customer whose broadband connection is being used at a given date and time.
How has this company got hold of my name and address?
They will have applied to the court for what’s known as a ‘Norwich Pharmacal Order’ or NPO. A Norwich Pharmacal Order is a legal procedure whereby it is possible to find out the identity of an alleged wrongdoer. We are  approached by the law firm’s solicitors with a list of IP addresses and then ordered by the court to provide the names and postal addresses of customers whose broadband accounts were assigned these IP addresses at a given date and time. You can read more about Norwich Pharmacal Orders’ here.
What should I do?
Since you have been contacted by a firm of solicitors regarding a legal matter, we cannot advise you how best to proceed as we are not qualified to give you legal advice. However, as your broadband provider we can inform you of certain facts regarding your broadband account.
If you are unaware of your account having been used for file-sharing, there are a number of ways in which this can have occurred without your knowledge:
•    Somebody else may have used your Internet connection to download the file. This could be a family member, a visitor to your house, or in certain cases even your next door neighbour!
•    You might have a wireless network that isn’t secured. If you don’t secure your wireless network then pretty much anybody within range of your router can connect to your network and use it to access the Internet. You can read more about wireless security here.
•    Even if your wireless network is secured, none of the commonly used encryption methods are 100% secure. Whilst it’s unlikely, there is a possibility that somebody ‘hacked’ into your wireless network and used your Internet connection without your knowledge.
•    Your computer might be infected with a virus or ‘malware’. Computers with malicious software installed on them can be controlled remotely and used to connect to file sharing networks.
•    The monitoring software used to identify you may be flawed, or an administrative mistake might have been made. Some people have argued that the monitoring software these companies use cannot be relied on, or the process of obtaining the customers’ names and addresses is subject to human error.
We are aware of cases where customers appear to have satisfied the law firm in question that they have not infringed the relevant copyright by offering to have their computer inspected.
I didn’t think you were co-operating with ACS:Law any more?
If you’ve recently received a letter then it’s likely to be related to information we provided some time ago. We only ever provided information to ACS:Law because we were obliged to under a court order. Given the way ACS:Law has treated the information we provided to date, we have suspended any further disclosure and if necessary will invite the court to review the conditions on which it orders us to provide such information in future. For further information please see the statement here from our COO. You might also find this FAQ on our website of interest.
Who do I complain to?
If you want to complain about the behaviour of a law firm (or any particular solicitor) then you can contact the Solicitors Regulation Authority (SRA). Contact details for the SRA are as follows.

Tel: 0870 606 2555 (Monday to Friday, 9am to 5pm).
Email: contactcentre@sra.org.uk
Post: Solicitors Regulation Authority, Ipsley Court, Berrington Close, Redditch B98 0TD

Further details can be found on some of the third party websites listed below.
Further Help/Reading
There’s a discussion about these letters over on our Community Site Forums here. The following websites are also very helpful and contain a lot of useful information. Please note that none of them are in any way affiliated with Plusnet.
Which? What to do if you're accused of file sharing
Being Threatened? Responding to an accusation – What to do if you’re accused of illegal file sharing
Money Saving Expert discussion forums
Slyck discussion forums
Consumer Action Group discussion forums