The Serious Fraud Office is investigating an earlier deal between Andrew Regan and the CWS on which Regan was advised by Alsop Wilkinson.
Under the deal signed on 25 January 1995, Regan's company Hobson Group paid out more than £5m to extend a supply deal with the CWS.
But the CWS received only £2.4m of this money, with the rest going to a third party, British Virgin Islands-based Trellis International, controlled by Ronald Zimet, who helped broker the deal.
Peter Wayte, former head of corporate finance at Alsop Wilkinson, now at Dibb Lupton Alsop with whom his firm merged last year, said he had not been involved in advising Hobson. Wayte refused to comment further or say who had advised on the deal.
During the CWS takeover bid last month, CWS questioned Regan. It asked him: "What did Mr Zimet do to earn £2.4m in three days?"
Regan replied that he and Hobson's board had satisfied itself with the help of its auditors, financial advisers and legal advisers - who were lawyers from Alsop Wilkinson.
According to the Financial Times, which obtained documents relating to the deal, Hobson was advised by Alsop Wilkinson on 24 January 1995 that there was no precedent requiring Hobson to disclose to the CWS the amount paid in brokerage fees.
Shortly afterwards two non-executive directors of Hobson, concerned about the payment, approached Allen & Overy - not Hobson's advisers for advice. They obtained signed letters confirming that neither Regan, Peter Hallett, the finance director, and David Lyons, Regan's associate, had benefited directly or indirectly from the £2.4m payment to Trellis.
Later that month Alsops, according to the FT, told the Hobson board that it was entitled to pay the £2.4m to Trellis and that "neither Hobson nor any of its directors, servants or agents is at risk of civil or criminal liability as a result of the payment to Trellis".