With a sharper mind this morning, I've stopped mixing my savings and add on costs up to realise that adding the Tech Pack (in lieu of my current standard DNS + voice activation), will give me DNS Pro for a mere £370 more.
It's a no brainer 
No brainer, I totally agree.
Regarding the extra cost, you are correct it is £370 when the 5.8" DNS with Voice Control is factored in and £420 when the 6.5" DNS with Voice Control is factored in, as the 6.5" DNS is £50 cheaper than the 5.8" DNS. Crazy.
Jealous that this was not available when I ordered my car. My only consolation is that I got a very good 3 year lease deal, never to be repeated I am told.