It's impossible to say from the distance why your playing is crap. Have you posted any recording?
Indeed it is, and I appreciate that. I haven't posted a recording because someone who can't play the piano sounds pretty much like anyone who can't. There are no surprises in store. This isn't "at 1:30 your left hand is a bit loud" - this is cannot play at all, it just sounds crap from start to finish in every respect, except perhaps the notes are usually at the correct pitch.
"pieces" means the first 8 or so bars anyway, because I don't see the point learning the rest until I can play the first 8 of something, or more often these days just scales, thirds and octaves and little snippets and so on.
Just imagine completely sloppy and inconsistent rhythm and pulse, pain in right arm, completely sloppy and inconsistent dynamics. Whether I'm playing a piece out of a noddy kids book like a nursery rhyme or a grade 6 piece.
The fundamental problem is, after reading / watching / trying I've reached the stage where I have no idea how to play a single solitary note. What to move - arm, wrist, finger, leg or at which joint. What not to move. And so on. If you read this forum, in summary, one person says "balance" another says "play it like throwing a stone" another says "arm weight" another says "build strength" another talks about arches, yet another suggests that you should just "imagine what you want to hear and play" - which actually works in a sense, I hear what I imagine I'm playing, but in reality that's all that happens, I hear the imagined sound, the real sound [as a recording shows] is usually worse than ever.
I think you need to be a really good pianist before you can actually get out of the piano what you're imagining you are getting with this method [but I imagine that the belief it works explains a lot of poor performances from otherwise good pianists] - on the up side, I can imagine drums and other instruments...

Nevertheless, I give them a try...and the more I try, the more and more depressed I get with it all. I've tried completely off the wall things too, I packed in smoking, stopped drinking caffeine, started walking / cycling. All in the hope that, even if there are plenty of pipe smoking, coffee drinking couch potato pianists around, if it can hinder to any degree I've removed it as an issue.
It's just completely bewildering. As I said higher in the thread I don't know anyone who can play locally to get a demonstration from. Those that have played have no idea about movements or technique either. Each one says "I just played - my teacher never said anything about movements or technique" but none of them got anywhere either, even the ones that passed a few grades didn't learn to play the piano. That's a lot of retards if Derek's right.
As you note, my arm says my technique is wrong. That could explain a lot about why it sounds bad too. I hope so, because that gives me something to aim at - trying to play without pain.
But, for that goal, I probably won't find a teacher in the UK to help. Why? Well, there was a girl on the TV the other day at some Royal Academy, on a Channel 4 series looking at kids with high IQs, and she had got tendinitis [and thus all the clips in the show had her playing noddy pieces]. So, if a prodigy in the UK attending the Royal Academy doesn't have a teacher to help her to play without getting injured, how will I?
Clearly piano playing has all but left the building here, at least for those of us without a natural ability to play without pain.
So I figure I'm on my own anyway, and I'm looking for that book, video or forum post that actually does explain what to do - there are plenty that have tried to explain, and more that have just talked around the subject, and perhaps some of these have helped others, but I've not found the answer yet.
You make me really curious to hearing you play 
Well, as I say, I doubt you'll hear anything that you won't have heard before. You certainly can't hear pain nor hear the cause of sloppy playing.
OTOH, there is a certain value of "curiosity" that means "I don't think you're as bad as you say" - and I do note a tendency from people that can play to state the opposite. Rest assured I am not being modest.