Sat Nav update - as if you cared.

Solved, and it didn't cost me a penny!...............
Obtained a hidden idrive service menu code from a BMW technician (who shall be nameless

) to access the software. Turns out there are 255 available navigation satellites hovering up there serving the geo-positioning of the vehicle's antenna.
Apparently you need a minimum of 4 or 5 of those satellites visible to the vehicle at any one time to ensure they triangulate it's position. I was showing I had only 2 to 3 alternating as "visible satellites" and consequently only had a 2D position fix rather than the necessary 3D.
It positioned the car somewhere near Leominster on the map!

Whenever I entered a destination, the nav planned a perfect route to it but obviously thought I was starting from that position. It was totally useless and the lady guide and heads up display were all over the place.
Reset the software (didn't need an update) and bingo! I've now got 12/14 visible satellites listed and showing on the screen. It's snapped back into a 3D fix with the longditude/latitude/elevation now telling me I'm in Porthcawl again.
Will see how it goes but it looks like I've saved myself at least the £150 "investigative charge" and probably a lot more because God knows what I would have been told at the dealer.