I'm considering the PSR S970 arranger as a replacement for my current controller (CME VX7). Has anyone used one of these with SONAR Platinum? and if so what were the pros/cons, anything else worth considering around the £1400.00 quid mark? I'm trying to make the right choice so any input/advice would be welcome. 

It would help if you gave us some idea of what exactly you're needing from a keyboard.  $2000 is not cheap -- and you're going from a fairly straightforward midi controller with no onboard sounds to an arranger keyboard with speakers and a whole lot of other tricks up its sleeve.  Are you going to be gigging with this thing? 
 
Are you a pianist?  You had 76 keys before, and the S970 has 60 - a noticeable difference, especially if you play the piano.  Again, some specifics about your background/intent would be helpful.

Yes , I find it amazing that Yamaha still finds a market for such old technology ( The Mall Organ one man Band etc) I would think that Band in a Box can do much more than one of those for a lot less money. 
It sort of amazing what Software can do these days so hardware seems redundent when it's trying to do the same thing. If you already have Sonar which can also do much more than one of those all you need is a simple controller. 
But as said, might be you have a reason for using one live or something.