Thank you for the reply Dexter.
The strange thing is that the control voltage at the mixer swings from a virtually zero value (0.2mV DC on key up) and a full conduction value at 245mV DC on key down.
The 245mV value did not change significantly when I reduced the R105/R104 values so it seems to indicate that the mixer diodes are fully conducting on Key down as expected.
Also something I didn't mentioned before: when I disconnect the wire between T4(3,5), that is the clock #1 mixer, and R105 I get no unbalancing of the mixer and a very weak transmitted signal as expected on key-up (or key-down for that matter as it is disconnected).
So it is as if the clock #1 mixer would be much more sensitive to unbalancing than the clock #2 mixer.
Scratching my head on this one.