He must have known something was amiss as he stopped claiming the ACA earlier this year "because interest rates had fallen so far that he was able to meet his mortgage commitments from his MP's salary" - an MP currently receives £63.291 per year (Members Series: Fact Sheet M5 - Members’ pay, pensions and allowances) which in McNulty's case is topped up with a Ministerial entitlement of £40, 759 (Members Series: Fact Sheet M6 - Ministerial Salaries), giving an annual salary of £104,050.
The Green Book 2006:Parliamentary Salaries, Allowances and Pensions (follow link from here) is the bible on such matters, and some relevant passage are posted below.
3.1.1. Scope of allowance
The Additional Costs Allowance (ACA) reimburses Members of Parliament for expenses wholly, exclusively and necessarily incurred when staying overnight away from their main UK residence (referred to below as their main home) for the purpose of performing Parliamentary duties. This excludes expenses that have been incurred for purely personal or political purposes.
You must ensure that arrangements for your ACA claims are above reproach and that there can be no grounds for a suggestion of misuse of public money. Members should bear in mind the need to obtain value for money from accommodation, goods or services funded from the allowances.
You must avoid any arrangement which may give rise to an accusation that you are, or someone close to you is, obtaining an immediate benefit or subsidy from public funds or that public money is being diverted for the benefit of a political organisation.