You could try sticking the RootMetrics app on your phone and see what that comes up with (basic online version
here available to play with in a web browser, stick your postcode into the search section and experiment switching between the providers)
Its not an exact tool by any means, but for an initial quick feel its probably ok.
My guess though, without looking at site above, just based on googol maps, given you seem to be in the middle of nowhere, is Vodafone is likely to be your best bet as they tend to have coverage where others don't.