After sending it off to get a new mic it still didn't work!
Video recording was fine but Voice Recorder or In-Call, NO.
In desperation I spent the best part of 3 hours reverting firmware and roms until finally I got the mic working again with good old CoreDroid 6.0 albeit a bit low/under water. Then I put the latest firmware on and hey presto it's working again. Still not for voice recorder but we can live with that.
(Makes me wonder how many microphones are there!).
Just goes to show that some issues may not be just hardware failures!