Sounds like they are out t ospin as much credit as possible out of you
I guess a lot of it depends on the size of the company - some large and huge companies have very rigid terms for payment which are set centrally, so if Johnnie in <remote county/country> tells a local supplier he will pay in 5 days it makes no difference to the central accounts function who only ever pay on 30 days.
Some small and not so small companies may simply say anything that will get the contractor to do the work . . . if the contractor says he will require payment in 5 days and this is what is needed (to be said) to get him on site then they will say it, regardless of whether they ever plan to pay in 5, 10 30, 60 or 90 days.
You will need to gauge yourself from the type of company whether you actually can expect payment in 30 days.
You should definitely mail/write to everyone you can think of (accounts, your contact, etc . . ) in the company telling them that the agreed date has already been missed, but you expect payment on 30 days - and give them a specific date. Then ring them the day after that and ask for the cheque number. If they have no cheque number then they haven't paid you, 30 days have expired and you have a struggle on your hands.
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