Nightmare client - how would you end this

If they are late with a payment then stop doing any work for them. Once paid then do the minimum to get the job over the line. If they ask you to take on a future project then up your price.

Get your payments first then end it!

Surely the lack of payment on its own should be enough grounds to terminate?

This isn't working for you. Sack them.

I’d just quit by being too busy for further work. Once they settle your invoices, be blunt if they ever ask again

Totally get rid - what's the point of working for yourself if you still have to put up with arseholes. It feels great saying no.

The unpaid invoices are sunken costs. Terminate your retainer for late payment and move on. The only risk I imagine is public complaint and negative feedback. If that's a concern, be more magnanimous about it. Say you think it is best for you to exit the project at this point for personal reasons, but you don't have share it with them. That personal reason is that you don't like them of course! I feel your pain, I've spent the last week bending over backwards for a client, only for them to reverse ferret on a decision with cost impacts and then lash out in all directions to apportion blame away from their own poor decision making. TBH, I'm glad the engagement can to an abrupt end, even if there is now some argy bargy over the invoicing.

Definitely get rid, they are taking the piss and likely know it. But to ensure you dont get in this predicament again I would reassess your pricing model. A flat fee is fine but needs to be clarified - this fee covers 20 hours of work which is based on how long this type of project typically takes with my clients. Once you go over the 20 hours, you will be charged my hourly rate.

You need to be paid up to date. They have not paid you yet for work undertaken
You don't need this negative impacting client
They are highly unlikely to change the way they operate
They are taking up too much of your time which is not reflected in your fee
They are impacting upon your other clients and work that is profitable

There are no positives here, only negatives.

Definitely dump them.

I’d give them notice, depending on how long the project is for. If it’s ending imminently I’d say no to any further projects.

If they ask, tell them that it’s financially unviable to have the smallest/lowest paid projects requiring the majority of your time and you will be concentrating on other clients.

You really, really need to learn when to fire a client, and it should have been ages ago with this one. Knowing when to ditch a bad client is probably one of the top three most important things to know when you're a freelancer.

Send them an email saying that you will no longer be able to service their account and best of luck to them. Short and sweet, no discussion or debate necessary.

get rid life s to short someone else will be along

I’d end the working relationship.

PP have given good advice OP but I also think your business practices need a review. A flat fee for project management is very low. Things always crop up and need to be dealt with, etc I have no idea how you've managed to survive so far!
You also need clear terms of business regarding amendments, timescales, payment , your right to terminate etc.
See a small business advisor if you need help with all this.

I would EITHER say 'I am sorry but I am fully booked for the foreseeable' or put them onto an hourly rate and charge them for the time they use.

I do very different work but had a customer who's demands were very time consuming & I did exactly the same work for many other customers who expected far less. I was a bit too soft to challenge if I'm honest but I just made sure I was too 'busy' to take on anymore work & they did relentlessly keep asking me (making me suspicious that they maybe couldn't get anyone else to take on the work due to their demands) BUT they eventually stopped asking. Phew

Absolutely ditch them and move on

Have you tried communicating with them about the issues?

Was in a similar situation - particular bug bear was checking up on whether certain things had been done all the time.
After explaining that normally most clients know that a professional would do this and only report if there was a problem and wouldn't expect blow by blow accounts of what was done, things improved.

They just may not be familiar with the industry and the 'normal' client behaviour.

Dump them! It’s always the least money you earn that is the hardest client to work with. I dumped a client that were micromanaging me (six directors and board members each commenting on fonts ffs) and I swear I felt like I’d gone part time it was such a relief to see that back of them.

I have coached Freelancers and my advice on working for free is to imagine yourself taking the money you should have charged out of your bank and just handing it to the Client instead of doing the work.
If you wouldn't do that then don't work for free

No pay no work surely.

Come on OP, as a project manager, this should be relatively straightforward!

Ask for immediate payment of the overdue invoice, suggest that extra fees will be payable for late payment if they do not pay immediately (this is legal - HMRC have guidelines), and these will increase with time, and stop all work until they pay up. You can say it all politely.

At that point I would say that you are unable to continue working with them, no excuses. If you need to, arrange a file with all the work done so far and handover notes!

I would simply advise there is a hold on further work until their account is up to date. I assume you were clear about payment terms and that they know payment for work from April May is outstanding, make sure you are fully billed up to date. That is the answer to every query. When your bill is paid , then i'd just terminate, if you feel you need to have an excuse, just say you don't have time to take on further projects. What i would not do is waste your time and effort setting out why. it's not like they will say we are a nightmare and we won't do it again. they won't become super organised suddenly. They will believe what the are doing is right it's their business and they have the right to stay on top of any freelance workers, you don't have the time for this, you are wasting more time chasing payments , there is no scope for discussion.