The issue with my phone is FINALLY resolved - but what a long and painful journey. Initially the phone came back from repair with the same fault, but also with a large scratch in the screen! It went back again for repair (much to my protest) and have finally picked a working unit up. It seems they've swapped the phone out as it was accompanied by a letter stating "we either couldn't repair your phone or it would take too long".
All software and firmware versions are the same as my previous phone, just IMEI is different. So yes, it looks like a hardware issue afterall as mic is working perfectly on this unit.