Advice on getting client to pay 5 month old invoice for £5000

Hi,
I would really appreciate some advice with how to deal with a particular client I have. I am a freelance animator, and have worked for this client several times, so I don't really want to be too aggressive with them as they have always paid (in the end) in the past. But it seems like the more my invoice is for, the slower they pay.
I am currently waiting on an invoice for £5000 from January. They say the reason they haven't paid is they have been waiting for their clients to pay, but they always say this. And I have to go on and on at them, and chase them every week, trying to get through to their finance department who fob me off, and dont return my emails or phone calls.
At the moment it feels a bit like a bully has stolen my lunch money and wont give it back.

What's worse with this job, is a friend worked on the job with me, and he got his accountant to chase it up, and within 2 days of his accountant speaking to them he has been paid. Which just seems to make their disrespectful attitude towards me even clearer.

They have this info on their PO:

"We will pay for the work when the client pays us. Whilst it has been contractually agreed with our client that they will pay within 30 days of invoice submission, in our experience this is rarely the case, and may take up to 90 days or longer. We undertake to keep all suppliers informed as to the cash flow status of the work. Invoices will then only be paid at the end of the following month from the date of receipt, provided that the details on the invoice are correct and the Producer in charge of the Project or Authorised Signatory has signed it off. "

Because this is in the PO Is there anything I can do? Or have I agreed to it by accepting the PO?

Any advice would be really welcome,

cheers

great, thanks james. i've sent you an email.

hi james, did your legal department have any advice in the end?

Thanks for providing information.