I would agree with J.Ryan.
If you are confident that the company has the ability to pay the full amount, a threat of an application to wind up should be enough. We have used this successfully in the past.
Are they an ongoing customer? If so, and they are a significant part of the business, consideration should be given to looking for security for any further credit that may be extended.
For that level of money, I would be looking for a Letter of Credit from their bankers.
As regards the account manager, that is a judgment call for the owners of the company to make.
How important do they value this customer?
Can the account manager be moved to look after other accounts?