Adding more audio decoupling around the oscillator and adding a common base FET RF preamp has totally fixed the common mode hum issues with my Polyakov mixer based DC receiver (WISPY) for 28MHz WSPR. Even running from a mains PSU I am getting good sensitivity and no hum. The board is working on the bench with no attempt at screening and appears totally fine. The oscillator buffer appears redundant, so has been removed.
Wispy Rev B (built using MeSquares)
One possible further mod is to make the input to the RF amp a series tuned circuit. This would be needed when the full transceiver version is made (back-to-back diodes would short the cap to ground on TX). For now, I will leave the little RX running to see what I spot.