He did commit a criminal offence. He recorded a telephone call made on a public telecommunications system (who initiated the call is irrelevant, and for why he recorded it is irrelevant) without either the knowledge or consent of the other party. It dates back in its original form to the 1947 Wireless Telegraphy Act, but wad updated and incorporated into the Telecommunications Act when the phone side of the business was hived off from the public sector and became BT.
An offence and a recordable crime.
I've arrested people and secured convictions for exactly that act.
The fact that he did it for his own use in a civil case may be submitted in mitigation, but it is not a defence. There is no statutory defence to this crime.
I'm not home so don't have access to my Blackstones Discs but this makes the point;
http://www.itccallrecording.co.uk/legal-summary.htm
Point 2 is the one pertinent to us, and doubly pertinent in this case as the gentleman was pursuing a business debt, the fees he was charging the PPI company for answering their calls etc.