Lol, it all feels like a mine field to me!! Thanks for all these links :) I had a quick look at the symptoms of APD, and the only one that DS has is he often doesn't hear in a crowded room/noisy background. My brother is dyslexic, DD1 is probably dyslexic and so I wouldn't be surprised if DD2 and DS are too, although it would be well masked by their speech problems.
DS has been diagnosed as having the phonological disorder because he misses fronts, backs and middles of words, not all words, sometimes all at once, sometimes not. Packaging would be ba a in He swaps sounds round in a word, so dog would be god . He adds sounds into words that shouldn't be there at all, so steam train is dea dain and timber is bur row 
. It is hard to get him to drop the extra sounds. His speech, not vocabulary, is I think like a 2 year old. It is so complicated, like learning a new language with none of the usual rules, lol! I have to decipher everything he says and translate to others, close family can get the gist of what he is saying, but strangers haven't got a clue. And, if he says something out of context, it is really hard. It really does sound as though he is slightly deaf....there is no definition to his words, but he's had a hearing test and a thorough examination by ENT, which confirmed a weakness in his tongue and lips. He can't lick his lips very well, can't control his tongue well and looks like he is 'chewing' his words.
He had the DEAP assessment done, should I ask for more assessments?
DD2 says things like additioner instead of conditioner, b sketti for spaghetti, crotted tream for clotted cream. She still has a lot of trouble sounding out words to read, as some of the sounds are alien to her.
I will mention APD to DS's SALT, and see what she thinks....we are waiting now for more therapy, hopefully more targeted to DS's problems. Do you think DS is likely to progress? I still feel like this is only the very tip of the iceberg, and there is a long way to go, but everything moves so slowly and I worry that it has been left too late for DS and he'll be stuck like this forever unable to express himself :( . He is such a bright, lovely, hard-working little boy.