Gd!
Genius!
I found where the drift was starting - around ic4.
This is what is in the documentation (pity I didn't read it before!)
"There are two other particularly important features in this design. The first is the method of drift cancellation which is applied
around IC4. Despite its mundane sounding number, the LM318 is a very fast high gain amplifier which can recover immediately
from large overdrive, and then amplify very small signals without overshoot or ?ringing?. To use it to its full benefit in this circuit
it needs to be d.c. coupled and operated at the highest gain possible. With this level of gain. battery voltage and temperature
changes produce significant output voltage shifts, which would mean regular re-adjustment of the frequency control."
I couldn't see anything particularly wrong, all my soldering looked good, no dry joints.
So I took the i.c. out of it's holder and used a little contact cleaner ......
It's now solid as a Rock!
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