This Deck being doing really well for me, good resources and card draw, being smacking down orks today without issues, and will continuing practising with it etc, looking for ways to improve it though as I dont like the dreadnought, and so far only used it a couple of times, any advice, here is current deck list- (mix of marines and Tau)

Warlord-
Ragnar Blackmane

I would drop the Bolters and add in some Iron Halo. And maybe find away to fit in Void Pirate or Rogue Traders.

I am not a huge fan of the Deardnought or Raven Guard Speeder but I am a huge fan of Darring Assault Squad.

Since you have have allready allied with the Tau I can only echo that the inclusion of Iron Halo would be a wise option (more for the Shield Icons than anything however).

This is what I'm working with, with great succes.

I used the World Champion 2014 deck as a template, removed Cato + signature cards and added Ragnar + signature cards. Stupid as it might sound, the deck is incredibly effective.

If you really want the Exterminatus in there (because it is a very powerfull card indeed) you could consider dropping the Promotions. On the other hand, I like them a lot because they'll allow you to continue to win the Command struggle or be used as Shields.

The main reason I am no giant fan of Exterminatus is because the card can be quite narrow in use, altough great, playing around it in the initial turns (by spreading your forces) is a play that is allready awarded because of the way how Command struggles are won in this game.