It would seem that it is perfectly legal for you to record your telephone conversations provided you make all other parties in a telephone conversation aware that you are recording the call for the purpose of clarification, substantiation and establishment of facts.

http://www.opsi.gov.uk/si/si2000/20002699.htm

http://www.seg.co.uk/telecomm/record.htm

Telephone conversations can be recorded in many ways, perhaps the easiest in this technological age is to purchase a telephone adapter (see links) and plug that into a recording device such as a digital dictaphone or MP3 player/recorder. There are a multitude of capable devices out there and the choice and expense is a matter of personal preference.