Sorted out... The problem was actually simple in the end... Horrible horrible Australian workman ship was to blame.

Although the tyres had being balanced i took them to another place and had them put on a new machine.

They were both out by about 90 grams a wheel... I suspect their out of balance aligned occasionally as i went around corners on the freeway etc and this threw the whole axle out...

A good balance all around fixed the issue.

Thanks for the help guys.