Hi John, I was thinking that there is a big difference in sound levels myself between the front mains and the centre.
So what? A "big difference" in what? What some setting in your AVR menu says, or what your ears hear?
Listen, Grassy. Your ears are the final arbiter. It is a lesson some take a long time to understand. I suggest an initial setup w/ Audyssey. It will set all speakers so they are the same volume at your microphone/listening position. Then YOU tweak speakers so they sound best to YOU. Why in the world would you default to some menu setting when something else sounds better to you?
My wife's chair is close to our Right Surround speaker. It was too loud for her, so I turned it down. We both had trouble understanding some dialog in TV/Movies, so I turned up the Center speaker until it sounded good to us. So what if it's 5dB louder than the Left/Right? That sounds like an Audioholic cartoon: Wife says, "I'm having a hard time hearing what they say". Husband says, "Yes, but the Center speaker dB level matches the Left & Right speakers".
All of your speakers and the fancy adjustments in your AVR are for one purpose... to make things sound good to you. Use your knowledge and tools to give you a good starting point. Then use your ears and judgement to make it best for you.