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As suggested by Nordcore (many thanks), I swapped the clock's IC chip for a TL071, installed a 2n3906 PNP transistor and a resistor which measured 4k74
There is no more bleed from the clock to the VCO pitch.
In the pictures, I hope you can make out that base connects to the diode and is conveniently marked with an arrow. The emitter connects to +15V via the 4k74 resistor, +15V being tapped from one of the slider's pins. The collector connects to ground via a short insulated wire.
I had already removed and bridged the component-side transistor
FIX for wrong Silkscreen for matched pairs Transistors in Filter section 
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