Hi Andrew
Most aircraft HF is on 5-6 MHz or 8-9 Mhz using USB (upper sideband)mode.
Shannon VOLMET is on 5.505 MHz and Shanwick Oceanic use various frequencies but 5.649 MHz is quite active.
There are scanners which cover HF but not sure how good they are - you would most likely need an external wire antenna for best results.
You can email me for more info.
