http://www.ofcom.org.uk/static/archive/oftel/consumer/advice/faqs/prvfaq3.htm
So you cannot use it as evidence in a dispute? As you'd have to play/transcript to a 3rd party (ie, someone else in the company)?
Clear as mud 
Like I said, you can submit a transcript of the conversation to the court but if the other side disagrees they are in effect accusing you of contempt of court. THEN the audio recording becomes admissible as evidence (dont ask me why it wasn't before, I am not that clever). It's a palava but fully legal, as long as it is YOUR conversation on your phone and you are not intercepting someone else's conversation which is a serious breach of privacy laws 
The logic behind it is pretty straightforward

As hopefully Messrs. English and Card explain eloquently below...
"Documentary evidence.
Documentary evidence consists of information obtained by the production of a document (or a copy of it, authenticated in a way approved by the court) as evidence of a matter contained in it. A 'document' includes, in addition to a document in writing, a map or drawing; a photograph;
a disc, tape or the like; and any film or the like. Where a police officer offers in evidence the confession of an accused which is recorded in a written statement, it is the contents of the statement which are offered in evidence and the evidence is therefore documentary evidence."
If that helps...