Thanks guys - & sorry,
Had a brainstorm & did something
completely different - but it works - no idea why? But good, clean levels in the software.
View attachment 95646
Don't know why it works - but it does - the trick was to a) plug the mic to the left channel, which I'd configured in the menu, then b) add another stereo lead to the set-up, this time between the Lexicon and the MG102c. It works, I microphoned through all the factory set efx and recorded the test at the same time - excellent sound quality.
The whining noise was caused by the mono jacks, when I replaced them with stereo jacks - perfect sound. Total mystery, I only have one stereo lead because I never use them, I had to cobble the other one together from bits and bobs :-o