Unpaid invoices

[Weedsnseeds1]

I am a contractor who provides an inspection service against an accredited standard on behalf of several different companies.
One of these companies owes me a substantial amount of money for completed services.
I have two invoices dating back to 2016 that are unpaid. The member of administration staff that dealt with invoices at that time left and despite repeated requests, the department that took over the payments haven't paid.
They have paid invoices since, but not these.
At the beginning of this year a new system was introduced, where a form has to be completed in advance of the work, guestimating costs, a PO is raised, invoice submitted, rejected as a mis-match, resubmitted to person who raised PO to amend PO number and sent back to me to re-submit to accounts. A complete faff and designed to delay payment in my opinion.
They have ignored all requests for PO since April, meaning that I am unable to invoice for the work.
It has got to the point where I intend to put in a claim to County Court.
However, please can someone advise if I can claim for work since April, as this has not actually been invoiced as they are preventing me from doing so by refusing to issue the PO?
So, strictly speaking they aren't unpaid invoices, they have been blocked from being invoiced.
They have invoiced their clients for the work I have completed, including my expenses ( flights, hotels, rental cars etc).
There is no issue with my work and it has all been completed on time.
Have they found a loop hole to get out of paying and also prevent me from claiming payment from the court?

You have wonderful leverage there! Write to them with copies of all outstanding invoices, stating that unless payment is received by X date you will be putting their account on stop. It's a fantastic way of giving them an incentive to pay

Bigger companies have a po system in order to track order /stock etc . Having worked in accounts for small company and a big company I see both sides . The po system should work to streamline payments although I can see in this it's not the case .
It simply isn't a delay tactic I suggest you contact company and ask to speak to someone superior, communication is key get a name for a manager and follow up with a strongly worded e-mail if you have names of senior staff including directors cc them in too . I'm sure it will get dealt with if escalated. A lot of the time old invoices are left if someone left and new person not sure what to do and told will be looked into. You need to force this to be dealt with email all correspondence signed pods person placing order etc deliver address .