Managed to resolve the key envelope problem with QCX 20 by replacing Q6. TX now all OK and envelope looks good.
However at 13v current is now 200mA versus (110mA before the Q6 surgery) and lots of noise in the headset.
T1 volts @ pins 5-6 (& IC4 pins 7 & 9) all 2.46. 5V rail is 4.91
C43-46 & IC5 all pins except 4 & 8 are approx. 0.9 V
The Q6 job involved some component bending but everything around R1 & R2 looks OK. Suspect IC4 but looking for some confirmation or less radical suggestions before trying to replace.