Thank you for replying Justafew
I had the volume on my phone set to the highest setting already including notifications, ringtone etc but still couldn't understand why there was no sound.
I have however just managed to solve it!
I went into Settings - Applications - Applications Manager and scrolled down to Messages. On clicking I found that notification was blocked! This must be the default setting so I changed it to allow and all is well with the world once again.
Thank you again for taking the time to reply