if you are doing work for them on a regular basis, stick 5% of the job price on top and at the time of invoicing, offer them a 5% discount if they pay the invoice within 7-14days (you choose what you think suitable.
If the agents have a bookkeeper/accountant that is paying the invoices you can gaurantee that you'll get your money on time as their landlord has already expected the bill of the full amount and any money saved on the job goes in the agents bin
5% dont sound a lot but when times are hard and agents are trying to make/save money then £50 saving on every £1000 will be well appreciated!
Good Luck