OK, good news! I replaced the volume pot with a 4.7K linear from my junk box and all my problems seem to be solved. QCX is now a pleasure to use and the Tx/Rx switching clicks are much less obtrusive to the point I don't really notice unless I listen, they were hurting my ears yesterday. I have five QSOs under its belt this afternoon so its well and truly working.
Comparing its power (primitive- just swinging a needle on an uncalibrated and inaccurate power meter) to my K2 set for 7 watts, the QCX was giving a little less so seems about right. I'll try and measure it more accurately next time. My Juma PA100-D has a fairly accurate power meter but I'll have to explore a properly sequenced switching signal from the QCX, I'm sure its been mentioned before so I will browse some threads and leave space in the box for an extra jack socket!
The pot I removed was erratic and jumpy when I tested it. I can't tell whether I damaged it or if it came faulty I guess doesn't matter, its fixed now. The new one is physically a bit too big so I'm planning to get one with a power switch built in. I had planned to use a little toggle switch but this might be a neater solution.
Onwards and upwards!