Late Payment Advice Needed

I work freelance, and have invoiced once of the companies I have carried out work for, for all work completed, however a number of the payments are now very overdue.
The company were supposed to provide me with a sub-contractor agreement for each individual job I was working on, however only managed to draw up these before the completion of work for 2 out of 8 jobs. For the remainder I either received no agreement (other than emails confirming rates etc) or received it after work was complete.
The payment terms were open ended (payment would be given to me once their client had paid them), so I was wondering if payment terms are allowed to be open ended ? (One of my invoices is 70 days overdue now, which seems an unreasonable amount of time).
I would also like advice on how best to proceed now, as when I email chasing up payment I get quite a rude response telling me that I shouldn't be hassling them for money and I will get it when they have it. Should I keep emailing regularly until all invoices are overdue, regardless of them asking me not to? And then If I have still not been paid for anything how should I go from there?
I have been told by them that as I agreed with the sub-contractor agreements I cannot request my payment, however as I only received these for 2 jobs, does the Late Payment of Commercial Debts act override what they are telling me?
Would appreciate any advice.

Hi David,

Thanks for your response. With regards to the jobs with no sub-contractor agreement, it was mentioned to me informally in emails that I would be paid upon them receiving the payment from their client. Also, I do believe that they have been paid by their clients on the jobs which my invoices are overdue (they have told me they have been paid in emails). Does this mean they are breaching their own terms?

I have been advised by Citizens Advice that I should send a letter detailing outstanding debts (including those not yet overdue), the date when I expect to be paid, and that I plan on taking them to the small claims court if I do not receive payment by this date. However, I was wondering if it would be best to do this when all of my invoices are overdue, or send this asap but give them a reasonable amount of time to pay up?

Also, would this be best to do via a solicitor to get the best result?

Thanks again. Amy