I have just QSYed to 630m (472kHz) from 6m. ERP is around 5mW (20% TX, 80% RX without preamp) using my homebrew transverter. I really ought get a better 472kHz antenna up for the winter. The earth-electrode "antenna" radiates, but I reckon a loop would be at least 6dB better. I not sure about a low Marconi.
UPDATE 1906z: F5WK (436km) was the first MF spot this evening.
UPDATE 2025z: PA0A (417km) has been spotted and has spotted me on 472kHz WSPR.
UPDATE 2315z: I find it hard to believe, but according to WSPRnet I am the only G station transmitting on 472kHz WSPR. G3XKR (347km) in North Devon has just spotted me.