Does the company close for bank holidays anyway? If so I would calculate 24 days / 12 months * 3 = 6 days accrued holiday.

Unfortunately I've never had to calculate where bank holidays are worked and individuals have the time off on another day - perhaps others have.

Welcome Sam

As is often the case with the breed,  your Finance Manager is pretty clueless about employment law.

The statutory minimum entitlement is based in between the two - on 28 days. Anything over and above that is a contractual matter: unless there's an express or implied right to it being based on 32 days, you can probably successfully argue no additional contractual right applies here

In reply to David:

PS Sam
Isn't 28 August a Bank Holiday?

Assuming the employee is off work for that August BH, you should of course deduct that day from the result of whatever pro rata annual entitlement calculation as above.