Your better off telephoning and asking for the accounts dept: Ask them if they have your invoice, and what their propcedure is for payment RE times and methods.
If need be then send a written reminder with a statement of thier account with you.
Genrally it's 30days payment (ha! ha!) from the end of the month. So if you sent your bill in on 2nd July it should be 30 days from the 30th July you getting the monies by cheque or EBACS around the 30th August.
But don't hold your breath many firms take longer to pay.
Did you not check the payment terms before doing the job? I would have done, but that doesn't mean they pay on time, checking with another contractor they use is best way of judging payment records