Just to clarify have they paid late 7 weeks running or not paid and now into 7th week of getting the service even though they haven't paid?
Either way if you're running a business, particularly one where you have employees you HAVE to be assertive.
My friend has a commercial cleaning company, she expects payment upfront cleared by close of business the day before the service will be happening. No money = no service. If they haven't paid she contacts them around 4pm to check in, if they're umming and ahing or giving excuses they don't get the service that week, if the same happens again the following week even if they eventually paid she drops them as clients.
As she herself says she's busy enough that
A they need her more than she needs them, she has a waiting list of potential clients, is always getting interest and knows she can replace them easily
B she doesn't have time to be chasing stingy arses who think they are too "important" to have to pay on time
C she has to consider her employees and making sure she can pay them AND stay in business
If you don't deal with this appropriately and stringently you're not just risking not being able to pay employees this month you're risking the whole business collapsing and putting yourself and your employees livelihoods at risk.
Credit control is a key part of running a business, contact the late/non-payers, the ones have really taken the piss cut them off, the ditherers put them on notice, but as pps say it may well be good to write to all clients with new t&c's
"Unfortunately due to issues with some clients over payment we are having to introduce a policy of payment in advance, you can do this by (whatever way works for you the business).
If payment is not received by close of business the day before we provide your service then we cannot provide the service.
We value our customers greatly and enjoy working with you, but without timely payments we are unable to provide the service that we hope you value too."