I've had this issue and fixed it last today.. go to settings voice control change the language from UK English to US English.
works perfect now.
hope it might help.
Glad sorted @Dean1969 , though not all Tv's will have this option in settings and will usually work ok if correct country set. Take you are based in US, is it american model?
A method for solving issues not otherwise fixed:
Turn off TV
Unplug from power point
Hold down the return and 4 colour button till remote stops flashing
Reconnect power plug to wall socket
Turn on the TV
Remote should then resync to the TV and if so the voice control will be active.
Problems are sometimes caused by server issues.