Many thanks for the reply Ignace.
The SHIFT + 3 certainly changes the setting form B to L or M ....(though I'm not 100% sure which off L or M is NTSC.
I have the satellite service Direcxtv installled and they installed 3 receivers in different rooms. 2 work fine (on my new Phillips HD tv and also a computer with Windows Media Center). The 3rd receiver was for my MX 4500.
Having changed the setting to L and done a frequency scan I find that I can get a black and white picture of the tv guide when the satellite receiver is in tv guide mode but as soon as tv guide mode is exited the picture disappears completely....... very strange.
To ensure that the TV powers up and runs ok I purchased an up-converter that converts USA power up to 220 volts so it is not a power issue. For some reason it is just not recognising / able to display the American picture or sound transmission.
My wife and I will be totally gutted if we don't get it working as we both love that tv.