[mole42uk]

Here's more information:

Both chokes are 6mm outside diameter, the winding is 40 turns, 9mm long, single layer, close wound. That seems to measure 34 swg?

Using a test setup with a 300Ω input, a 3.3n capacitor to ground on the input and an 8.2n capacitor to ground on the output terminated at 50Ω.

The air core resonates at 1.61MHz, the slugged core resonates at 1.32MHz with the core in the centre of the coil. Perhaps I'm using the wrong word here, the pi filter peaks at these frequencies - is that resonance or not?

Please don't be put off by my mention of the 10µH choke earlier, that is one I was using until I found that its resonance was too low for this circuit.