The first question is - "is there any goodwill left between you and the client?".
If there is, then you are going to be far better off dealing with him on a goodwill basis than going for it legally, and so on.
I occasionally fucked up like this: failing to be "businesslike" about costs upfront, taking ages to invoice, and so on, and it does make it a lot harder to then suddenly be businesslike when the client's got used to you bumbling along kinda doing a bit of work here and there and not asking them for money!
You've got a bit of an uphill road ahead of you re-establishing your businesslike credentials, but you need to do it if you're going to carry on in that game. So try and get him onside. Accept that you make have to take a bit of a hit on your invoicing.
Assuming that he's not just out to rip you off, and that there's any future in this business relationship, this is what I'd do:
- Agree with him an amount for the work you've done and submit an invoice.
- Agree with him that, from now on, you'll quote and specify all work in advance, and invoice each job regularly or at least consolidate small jobs on a monthly basis.
- Whenever he asks for a job, co-operatively (not obstructively) explain that you'll need to take a short while to take a look at it and see what it entails. Then do that, and send him a written (or, if you must, an emailed) quote. Be sure to add a contingency factor of 10-20% on to cover your "risk" - ie, the risk that the job has unforeseen complications. If you feel the situation warrants it, also bulk the job up a little to reclaim any losses you've made on the invoice you're waiting on at the moment.
- Invoice promptly. No, really, do. It sounds counter-intuitive, but actually people like to know how much they owe, and it puts you in a much better position if you're going to argue, because the issues are fresh in everyone's minds.
- Chase for prompt payment. It looks good. With some of my more recalcitrant customers, I started adding on 2.5% then offering a "2.5% prompt payment discount" if they stumped up within 14 days of the invoice date. With about half of them, it worked. If you don't want to do it, bribe a girlfriend or associate to do it for you.
ETA: I've read the rest of the thread more carefully now, and it does sound to me like you have gone to quite a lot of trouble to address the first point I made above, but it doesn't sound like he is being reasonable.
What would it take to restore the site back to the condition it was in before you did the work that is currently in dispute? Because I think you're probably getting close to the point where you may want to do that. Save embarrassing him for when it is clear that nothing is going to be forthcoming, but as an initial shot across his bows, taking the site back to as-original condition and saying "right. NOW, to make said updates to system...£x" might be the way to go. It's starting to eat into any goodwill, but if he's being as directly unwilling to pay the going rate as he sounds, maybe that's gone already.