Update on our aircraft. Firstly thankyou all those who responded to my inquiry. After finding that the responses were split about 50/50 as to if my LAME was correct or not I decided I would contact CASA direct and spoke to an Airworthiness Inspector.
The result of which was that he was correct. Apparently it is more to do with weight and balance effects than the fact that 1 comm is u/s. The mere fact that the radio probably weighs about 1kg and is pretty well right on the datum line makes no difference. Them is the rules and CASA loves rules. So unless my LAME can source a loan unit we are grounded until ours gets back. However we can ask a CASA delegate for a permit/engineering order to allow us to continue flying while it is out. Depending on the cost of this apposed to the time to getting the repair done will decide which way we go. Our aircraft is only private category so no business pressures there.
So well done to those who said our LAME was correct and to us others who thought he was wrong we have learnt something today. I still think it is regulation gone mad but who cares what I think.
Thanks again
MT