My theory is surely correct: the MF noise floor is rising and my 5mW ERP signal is becoming much harder to see unless the RX station has a low noise level, either as a result of optimising antenna and antenna position or good luck. G8LCO (58km) who always gives me a good report uses a well positioned E-field probe antenna. Overnight best DX on TX was LA3EQ (769km) but his report was -30dB, so "only just".
I am seriously thinking of abandoning 472kHz TX until I can get the ERP well up by getting a better antenna up in the air. I shall try 472kHz RX without the SMPSU. Turning this off gives me a 7 S-point improvement on RX!! Without the SMPSU charging it, my PC battery should manage a night. Certainly several hours.