Today I've been resonating my receiving loop to see if this improved things. Well, I tried it - no better. Then I improved the active high pass (50Hz) filter which got the mains hum down some more, removed an audio gain stage in the RX just leaving 2 common emitter stages. With this, 4W TX, and the loop resonated around 2kHz I did a further walk-about test. 0.42kms was the limit of range. S/N better and 50Hz hum not a problem even close to cables. The RX needs some more gain and narrower selectivity.