Thanks for your answers, I should have said that I did start a thread in SN, where I have had some enlightening answers as well.
There are some similarities with your DD Marne and yours LeonieDelt. Whenever we talk to a health professional about it - eg SALT, Deaf Advisor, paediatrician - they never really talk about this specific issue and its something that preys on my mind.
When DS does come over to talk to me, it is mostly about numbers (we count a lot or say numbers in funny voices, which he thinks is funny), or he wants me to repeat phrases that he thinks are funny (eg a line from a book) or wants me to do something like get him a box. If he wants the box he will say "box" then "want box" then, "what you want?" box".
If I ask him a question, he rarely answers. Although, I did ask him what he had for lunch yesterday when out with DH. He took ages, and then when he had his back to me he said "chips". So that was good.
I do do similar with the yes or no thing, but he repeats "yes or no" most of the time.
There are a number of areas we have to work on a the mo, his pretend play being one, so I think we will knock potty training on the head as DS just gets upset although he has weed on the potty a few times.
Thanks so much for your responses, they have been reassuring. And thanks for reading my ramblings. I never get satisfactory answers about this so it has been good to write about it here.