How do you mean no engine noise? Does the starter just click / or just nothing at all?
Do the lights come up on the dash normally when this happens or go dim when you try to start?
In answer to your questions -
the starter makes a click but that is all. Lights come up on dash as normal but haven't noticed if they go dim or not.
Have used car this morning and has started first time
four times without failing once !
Best to start of with the basics but being intermittent would make a little harder to diagnose - But id imagine the best place to start any given situation would be the main wiring to starter/alternator and engine ground points/straps. as and when the fault occurs check the voltage present at the stater, if you have 12v at starter while trying to crank - signal to the solenoid needs checked out . if found to be below 12v while cranking, trace the circuit backward testing to see where the 12v went...have they inspected the starter relay.might be best checking or swapping that..another common fault these experiance is ignition switch wear..and this can sometimes cause non start issues so they may want to look at that...