Got a German made disconnect from the Malt Miller, and guess what?- no flippin' difference! So I thought it must the corny keg post that's causing the problem. I swapped the 'gas in' post for the 'beer out' post and it works fine.
Actually, I don't use the 'gas in ' post, because I fitted an S30 valve in the lid and use this to pressurise the keg, so as long as I mark which is which, I should be ok. Can't see anything wrong with the faulty post ( it keeps pressure and you can open it with your fingernail). I think it's maybe too big in diameter as you can't push the disconnect down far enough to release the beer. I bought the keg from Kegtopia; it's one of their 10L new kegs. Anyone else had similar problems with this kit?