CakeAlexS
It's very much an individuals choice IMHO on where you end up spending the money... I would subscribe to Groove3 myself.

That's a most excellent piece of advice.  And if they (Groove 3) are true to form, you'll be able to get an annual full access one-year pass in Nov. for 99 bucks.  I'll be signing up for the third year straight if they deliver on the special price once again.
 
The other advice is to just be patient and watch the marketplace.  I've gotten some outrageously good bundle prices in the occasional group buy or special sale.  I picked up everything (at the time) in Nomad Factory's catalog for a hundred bucks a few years back.  There was a recent sale of the Plug & Mix VIP bundle for 49 bucks recently, if I'm not mistaken.  Now, not everything in either of these packages is useful or even very good.  But there are some gems in both, and for the price, these were wonderful buys.  But don't be in a hurry.  What came with SONAR should keep you happily mixing for the moment.

Thanks guys, lots of interesting views and ideas!

You can get great deals on Ik Multimedia's T-racks Deluxe at sites like KVR. I got it for $69 and all processors work as single plugins. I would suggest you buy Cakewalk's Pro channel stuff, they are very good IMHO. Fab Filter's mastering bundle is very nice, but If your on a budget look at Toneboosters, Klanghelm and Hornet. For a very nice Limiter try the free LimiterNo.6 and Loudmax.

OK - I know you are interested in adding to your stable of plugins, but I'd like to first discuss the song  "Shut the Door".  You and your musical partner(s) are talented - this is very cool tune. 
Now - regarding software - if you gotta get something, I will echo those who suggest Ozone.  It's a quick easy way to
try a lot of different treatments (using the presets) and then focusing in on those you find beneficial.
IMHO - if you really want to "up your game" - I would suggest you instead critically review your monitors and listening (acoustical) environment.  Often the most benefit is made from really studying reference tracks that have the production you are striving for, and then attempting to emulate it.  This process works best in a good listening environment.  Bottom line:  decent monitors in a controlled acoustic environment....  ( Iknow it's a boring answer but really is important  )

Bit late to this discussion, sorry.
I have ended up with Melda SpectralDynamics because I was trying to use the LP64 as a dynamic EQ but it just wasn't quite cutting it. Also, the Waves DeEsser because it has a few more tweaks than the VX64 vocal strip. I've tried a few other Waves plugs and I have the free Melda plugs but I always seem to go back to Sonar's plugs and Pro channel, as everyone seems to agree, they do everything needed, very quickly and very well at that.
I don't have a dedicating mastering suite, but then again I don't think I have the listening environment or the need yet, I tend to outsource any songs that need mastering to an engineer that I have known for a long time and trust his ears and equipment better than my own. It's also good to get a fresh set of ears listening to my mixes because by the time I've finished with them I am too invested in their sound to make any drastic changes.
I guess I'm agreeing with those that are saying, look for a need and go from there.

"so I have around £200 (GBP) to improve my set-up" 
 
I still don't know what that funny cymbal is in front of the number 200?

Thanks to all

I decided to go for the Izotope Mix (Alloy 2) and Master (Ozone 5) bundle

I got a great deal and also got the free upgrade to version 6 of Ozone - although the biggest benefit is having both as I still find myself drawn to Ozone 5 but I will continue to play with 6

Alloy 2 is also really good - the transient shaper is a joy to work with and the EQ (like Ozone) is really transparent and sounds different from the Pro Channel EQ so I have a different option for EQ

All in all I am very happy!

Jablowmi19 it's a Zildjian