Well, after nearly a couple of days of using Radarbox, here's my polar diagram. Note the YELLOW is my Radarbox coverage, and the RED is my SBS-1 coverage from a few weeks ago. You'll see that coverage is similar. I really need to leave this running for a few more days to get a full polar diagram.
Unfortunately my coverage is restricted in some directions due to being surrounded by houses, whereas I live in a bungalow. By overlaying my polar diagram on a street plan, you can see where some of the houses restrict my coverage. I guess I need an antenna up on a 10m pole.
As far as network sharing goes, I still have more sharers on Planeplotter than I do on the Airnav network, so Airnav really need to sell a Radarbox to someone on the Western Isles or the Shetland Isles!