Payment of invoice at the end of a contract

Hello,
I was hoping you may be able to offer me advice or information on an issue I am currently facing.
I have recently been given my months notice that my contract with a client is coming to an end. I still have an invoice which is due to be paid. I was given verbal confirmation that this would be paid before the client went on holiday (it's a 3 week holiday and won't be back before Christmas). As this payment term had been agreed I shortened the number of payment days on my invoice.
Since then I have not been able to get any sort of correspondence from the client who is not even acknowledging receipt of my initial email with the invoice. There is also no auto out of office message to imply that they are not checking their mail.

My question is this... which takes precedence over payment terms, the terms you set out on the invoice or the initial contract for work which states that invoices will be paid within 30 days?

If anyone has any advice on the best way to try and get this issue resolved, I would be very grateful. I want to continue to conduct myself in a professional manner even though the client is not.

Many thanks