When you get the job, price it up. Let's say it's a £100 job.
Send them a quotation that prices it at £200. Put on the quote that you offer a 50% prompt payment discount for payment within X days of invoice.
If they don't accept it, move on, as they are quite clearly not intending to pay you promptly.
If they do, send them a gentle reminder seven days before the price goes up clearly stating that they have entered an agreement to pay you the full £200 if it goes past the discount date. If they don't pay, chase them and if necessary sue them for the £200 and refuse to accept less.
I've done it three times now and it's been accepted every time. And up to now (one job not yet reached the trigger point) they have paid a week or so before time. I think that what happens is that the person signing off for the job sees how reasonable it is. For all I care, their accounts department may have conniptions but that's not my problem.
Vin