Invoice Etiquette

I know I have posted on this topic before and in my recent post (Sinking without a Trace) I have pledged to "grow some" and stand up to clients who are taking the proverbial but...

I have two clients for whom I have completed the accounts, mostly and now they have the information they wanted (in one case an estimated Net Profit for a mortgage application, and the other the amount of corporation tax due for the year), they are refusing to respond to emails sent requesting a last few pieces of information required to complete the accounts and thus raise my invoice. I know I should be billing up front and collecting Direct Debits and I am tackling this client by client. But in the meantime:

1. For the client who has 2 small queries outstanding and has his mortgage reference, but the accounts are not completed would it be acceptable to send him an invoice for a % of the work completed? This was a substantial job and because the mortgage reference was required asap I (foolishly) prioritised it. It's been several weeks since the final review meeting which highlighed a couple of small points that required clarification. Otherwise 90% of the work is completed.

2. For the client who wanted to know their corporation tax bill for the year, they again wanted their accounts completed asap. I attended a year end review meeting, which one of the directors decided not to attend at the last minute, but the other director happily signed off the accounts. The following day the other director (who could not attend) sent me an email with a list of queries and has requested that the accounts were not filed until the issues were resolved. This has been rambling on for 2 weeks now, and the invoice I raised (the day the accounts were signed off) is overdue for payment. As the accounts have not been filed, is it acceptable for me to chase for payment?

I will be insisting that both clients pay monthly in advance next year or seek a new accountant as I'm sick of dealing with these people.

Thank you in advance for all advice.

SD