After a few days of data I've discovered what I can and can't do re data. It would appear that the E-Ham reviews on the EH Arno Cobra are correct, insofar as rf radiating from the feeder is conscerned. If I try upping the power out from 5 watts, I end up with a distorted signal. However, as 5 watts is about the going rate for wspr I don't suppose that's too much of a problem.
I have made made a few contacts on 40 using jt65-hf with the 5 watts and Arno ant, but as 20 metres is much more lively with this mode I migrate there during the daylight hours and turn the power up to a respectable 10 watts. The Pro Whip has no problem with that, and 10 is about right for that mode I think.