Update. Just chatted with a Sony representative. This is the worse than useful help I got...
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Agent: Hi Richard, thank you for contacting Sony Xperia UK Chat Support. My name is xxxx, please could I take your full name and email address?
Richard: Richard xxxxxxxx
Richard: My email address
Agent: Thank you for providing me with this information, Please could you confirm which model of phone you have and which network it is on?
Richard: xperia z on Tesco mobile
Agent: Thank you Richard, how can I help you today?
Richard: I already typed my question - did you not get it?
Agent: Sorry Richard, was seeking clarification. So you are not getting the chance to use Google Voice Search when opening up the application?
Richard: No. It works a couple of times but then, as soon as I touch the microphone icon, it bleeps and says it didn't catch what I said. To fix the problem, I have to either force stop the app or re-start the phone. After a couple of goes it will do the same again.
Agent: To confirm, you have also uninstalled the updates of the application Google Search as well?
Richard: Yes. It starts to work fine when I revert to the older version. This is OK in the short term but a pain as I have to make sure it doesn't get updated when I perform other updates.
Agent: It would appear to be an issue with the updates that Google have released for this application at the moment so can only advise what you are doing for the time been until this has been corrected.
Richard: Not sure yu should be blaming Google as the update runs fine on my other 3 Android devices.
Richard: There is clearly some sort of clash with Google Search and your firmware
Agent: There is nothing more that we can assist with at this moment other than what you are doing for now as it is not our application.
Richard: With all due respect, hardly any of the software on any given Android phone is 'Yours'. Google Search is a fundamental part of any Android phone and it should be working. As it seems to be fine on every other device, it would seem there is something in the Sony firmware/software that is causing this to happen. I was hoping for something a little more concrete than just having to grin and bear it. Especially on a ?550 phone.
Agent: As said, there is nothing more we can advise for you at the moment in time than what has been advised and what you are already doing. Sorry for any inconvenience this may cause.
Richard: Right. Not much help at all then. Sorry to have wasted your time.
Not so much rude as just totally disinterested. Thanks Sony.