Seek legal advice, one important question is how did this independent solictor obtain your details? This could actually be a scam - demanding money and then promising to go away does not sound like the normal letters. Generally people get a warning letter from their ISP asking to confirm things.
If wary of cost of a solicitor the Citizens Advice people are worth a chat to.
No idea where you are in the UK but
http://www.stoke-cab.org.uk/ gives the format of the URL's they normally use, and
http://www.yellowpages.com/ will probably produce a telephone number in your area.
I would also advice using google to see if others have had a similar letter and what has been said.
Andrew Ferguson,
andrew@thinkbroadband.com
www.thinkbroadband.com - formerly known as ADSLguide.org.uk