Dont let it go and tell the whistle blower that he should inform the cabinet member/councillor for housing in your local authority. Within most councils there is likely to be an overview and scrutiny comittee who review the financial aspects of the way in which the ALMO works.
If there is a financial irregularity they can compel the ALMO a) to reveal what it is b) State what action they took to reveal it and c) chjange procedures so that it doesnt happen again.
Right is on your side. Best wishes