I have just received a pdf document from the ex-treasurer, Russell Kelly, fully detailing the circumstances.
Very briefly, Russell feels that the accounting method used on the balance sheet (and in practice) should make provision for, and recognise, future liablities, especially with respect to 2/3/4 year memberships paid in advance.
He is seriously concerned about the solvency/insolvency of AOPA with respect to this.
He was unable to reach agreement with all board members over this and felt it necessary to resign.