MikeAqua: Cheers for the tips. 

I have tried removing the splitter and substituting a connector - it doesn't help. 

Just had a thought about the antenna though.   Does it matter if the long axis of the reflectors is parallel or perpendicular to the elements?

Definitely.
Horizontal
http://api.ning.com/files/d*-7kVWFfllPbIs2QaceOKNtXO-5sOT-fklyB1fXX5DK7w9-WMt1cH29RBG0Fcl*YaLbOKKheTrtUFHNY*hqi-yojOLjsCO8/HorizontalPolarity.jpg


Vertical
http://api.ning.com/files/3AUW0abOoehh2WjxSi3bSI2K86y*HTeflNz6eW0lZmaujCrRtX5wwIVit*VwMU4KTA5BNVZsEB82AZCFvVjV3NOTYeX4UCjC/VerticalPolarity.jpg



Yours should be vertical.