An update for those who asked about the outcome...I have decided that 3 failures in 30 days or so is about 2 too many ...no matter what the reasoning.
FWIW I do not buy the "static" explanations pushed by Airnav ...if this was so ,most of the equipment I have here should have died a dozen deaths- and it is apparent that the competition does not have this problem.
SO I have returned the RB to the supplier and been refunded....
For the moment I can do whatever I need using Planeplotter and MLAT as my primary interest is Military aircraft....and there is excellent SBS coverage in this part of the country.
Maybe in the future when Airnav have managed to make this thing a lot more reliable I may be tempted again - it is after all said and done a nice piece of gear but , quite apparent from this forum , some boxes suffer from an ongoing fault - maybe the SAW filter - maybe something else...but whatever it is there are obviously quite a few instances of the problem coming to light.
I would add - that I wonder how many "new" users , not knowing what to expect, are in fact suffering from the "deafness" problem.
Personal opinion only - but I feel that the problem is possibly more to do with Quality control than anything else(on the 2nd batch boxes)- and the place I would look to would be the soldering quality during assembly - many of the defects reported here seem to lead back to "dry joints" on the USB socket and SAW filter...I believe the box is assembled using lead-free solder - which is (again I believe) more prone to dry joints.
Whatever...it's not all brickbats ...a bouquet to W&S who both times they had the box turned it around very quickly ...in particular to Gary in their service dept who took the trouble to phone me the 2nd time and tell me it was on it's way back.
Thanks also to all who took the time to answer my questions here - including Airnav support- I'm not getting at you ANS - I just don't believe the "Static" explanation is the complete answer.
DAve