Ultegra Upgrade Assistance Req

[Vandiesel]

Hello all,
I'm looking to upgrade my standard 105 rear derailleur to an Ultegra in dark grey. Please could somebody clarify if I would need this version
http://www.wiggle.co.uk/shimano-rd-6700 ... #tabBuyNow
and can I run with a Tiagra cassette 28-12 or would you advise me to upgrade this component to at least a 105 version or can I still use the Tiagra?
I have already upgraded the front chainset to one of these
http://www.wiggle.co.uk/shimano-ultegra ... -chainset/
53-39 172.5.
Any help would be brilliant please,

Thankyou,
Andy

 

[Vandiesel]

Thanks for that Beano, would you say the 12-28 is prob the most common used?
Andy

 

[Vandiesel]

Thanks mate, very grateful