Apologies for my rant in my initial post, I think I was a tad angry that day ! Anyway, after scouring the web for weeks and ultimately spending an hour on the phone to UK HTC Support, it turns out that it's part of the ROM build and cannot be disabled.
The only way around it is to root the phone and disable it that way.
Unfortunately, that isn't an option for me as I'm stuck between a rock and a hard place as - I absolutely need the Orange Signal Boost app that comes with the Orange branded stock ROM (See my other thread about this !).
How rude is that though ? HTC forcing their navigation app on people, whether they want it or not, and giving them no way at all to stop it constantly popping up and asking you to buy a licence ! Out of order HTC.... out of order !