sole traders customer has still not paid invoice or replied

Hi, sorry if this question has already been asked. I'm new to this site. Anyway...

I'm a self employed painter and decorator and I have recently carried out a weeks worth of work for x amount through a property agency. They had a flat that was going on the market for sale and the home owner wanted it painted. Now, like all my customers I type up a headed quote detailing Labour and material charges in a simple format along with a small section at the bottom that states "agreement" and mentions that payment must be made within 5 working days of the invoice date or a late payment charge may be applied (I didn't say how much/% rate). I've never had any problems with later payments before or with previous work through this particular property agency.

However, it has now been around 8 days since the invoice was sent to the property agency who forwarded it to the home owner. Since then i have asked the agency to follow up payment after 5 days since the original invoice was sent and I have still heard nothing back. So I'm starting to get a bit worried. All work is complete, invoices sent, no disputes regarding quality of work, bank details are on the invoice etc... it's simply an overdue balance.

How would I go about adding interest (daily rate etc) or late payment charge? I'm also in Scotland so I'm not sure if that would be an issue. Think it's something I should have had in place a long time ago.

Thanks in advance,

Michael

I sent my invoices via Joist for the customers to sign but none ever do. Do they have to sign the document for them to agree to your payment terms. For example mines states on the top of the invoice that it's 30 days. I also state that all materials remind my property until paid in full.