So far, spots have now been received from 10 unique reporters (9 via the WSPR database + G3YMC's report using WSJT7 not shown here) in 3 countries and 7 grid squares.
If I could summarise what this tells me it is:
(a) WSPR works very well at 500kHz allowing weak QRPp signals to be detected.
(b) Very low power (ERP <0.3mW) and electrically tiny antennas do work on 500kHz if weak signal modes are used.
(c) 136kHz is worth a go next, even with the poor antenna and very low ERP.