I tried shutting the power off to the internet radios today for 2 hours while there was a dropout - unfortunately no change.
I then tried taking the emon base to the emonTx and this did not help either.
So a friend of mine with an antenna analyser had a look at the feed and the antenna and helped identify that the stub antenna had a faulty connection in it. We installed another smaller stub antenna that we had handy and I will assess the performance over the next two weeks. Current receive strength is about -73dB.