Resources useful for tracking and reporting spammers

ASA non-broadcast CAP codes The Advertising Standards Authority rules are similar to the ICO PECR laws, and you can report spammers to them. Rules typically broken are in section 10, in particular 10.4.2 (not unsolicited), 10.6 (not anonymous, opt-out) and 10.13.3 (similar products only). Code 10.3 specifies that the advertiser must tell you where they obtained your details if you ask in writing.
aql phone network lookup Put in a phone number and it will tell you which phone network it came from so you can report it to the phone company. Home users can take their numbers to other networks but text spammers will send from the original network until stopped.
CAB's advice on Small Claims Court Citizen's Advice has information on how the Small Claims court works and how to use it.
Company Check
Companies House
Once you have a company name then put it into these sites to find more information e.g. the registered address for the company and the directors. Look for the directors names to see what other companies they run.
Data Protection Act This is the UK law which defines how personal information (such as phone numbers) can be used, and what responsibilities processors of such information has. Of particular interest is the Subject Access Request section as this can be used to force marketing companies to tell you where they got your details.
Data Protection Registry Anybody dealing with personal data must be registered as doing so as per section 17 of the Data Protection Act 1998. If they aren't listed in the registry then they are breaking the law and can be reported to the ICO.
PECR The Privacy and Electronic Communications (EC Directive) Regulations 2003 defines the laws which text spammers are breaking by sending unsolicited adverts. Of particular interest are sections 22, 23 and 30 as they define how text marketing can be done, and specify that damages can be claimed.
Domain Tools
Network Tools
DNS stuff
If you have a web site domain then put it into these sites to find out who it's registered to and how they are connected to the internet.
Google Just put the source phone number into Google to see if anyone else has reported it. Also try the company name, postcode, director names etc. to see if they bring up any useful connections and/or previous reports.
Spam Legal Action Anti-spam site set up by Nigel Roberts after he won a case against and E-mail spammer. Lots of interesting information there.
Who Calls Me
Who Called Me
These sites allows are great for researching phone numbers. Just put the number into them and see if it's already been traced. Remember to post any additional information you find back to the sites for other people to find!