I just saw a post by a clever person who discovered a wiring error in the wiring diagram.
I am using one of those universal case kits by inkits with the extra front mounted circuit board for Ptt, audio and CW.
The volume high wire that goes to the volume pot is not the black wire, it's the brown one. The connections diagram appears to be incorrect. I have swapped the two wires and the problem is fixed. I think the post was on Facebook but I can't find it again, so whoever that clever person is, thankyou!