I know the feeling - a lot of my client database is receivership property so waiting up to six months for payment isn't unknown although every three months is more usual. In many cases, I've retained the original contractor and added a percentage to reflect the erratic payment terms so that I'm effectively charging to lend money.
Situations like this can get into a theoretical 'battle of the forms' - the legal test to see whether it's the buyer or sellers terms and conditions which form the basis of the contract- but the reality is that customers will generally pay according to their process and that this is especially true when it's a large company employing a small one.
Is the job of significant size? Although I'd recommend fairly strongly against it, invoice discounting (or factoring as it's called in old money) might be an option if this is starting to strain your cashflow. The only alternative option which I can think of is the one which I suspect you're employing -being the supplier who gets paid promptly since you're going to be on the phone daily otherwise.