I have tried altering the value of the resistors to 100k and 1k, and there isn't any difference. The attached image is a screenshot of the oscilloscope just after the filter. If I remove the gain, then it works fine. If I add the gain, I get a signal like the one in the image.
Also, I think that if I reduce the gain to 10, so 100k and 10k, the oscillation goes away. I think so anyway. Tried so many combinations.
Cheers,
Phil
p.s. Oh and I redesigned the test pcb. No ground or other signals around the Hi-Z input, so hopefully, no parasitics.