I'm recording directly into my UA25EX (USB) with Producer 8.5. However, I'd like to add a mixing desk with faders to my system.
 
I'm wondering if I need to source a specific type of mixer or what would you recommend? Is there anything that will control the faders on the Cakewalk screen console or can I just use any analog mixer? If so, how do I link it up?
 
Big thanks  :)

There are controller interfaces that will control Sonar. These could be a USB controller or a full on digital mixer. Analog mixers generally do not send MIDI or controller info but are less expensive than a digital mixer. If your interface has only 2 inputs an analog mixer can be handy for allowing a lot of stuff to be always plugged in and ready to patch. You can also utilize one for controlling your monitor set up. But analog mixers are not controlers. Some might have USB conectivity but this is normally just a stereo in / out. 
 A digital mixer goes one step further and can control your DAW too. It can also use the SPDIF or ADAT outputs to bypass your interfaces A/D converters. A lot of digital mixers are fully capable interfaces. The price just keeps going up as the features are added. 
 
I use an old Yamaha 01v for my system. They are super cheap to pick up, Theres an guyy selling 2 for $400 on the Yahoo user group forum. 

Thanks for the advice. I recently purchased a Presonus Audiobox 1818VSL which I'm not using but intend to. Do you know if I can use a mixer such as your Yamaha with this (or the UA25EX)? If so - any advice on how I should link them up?  Cheers.  :)

Any mixer should work fine with the UA-25.
 
For myself, my mixer went into storage 8 years ago when I bought my MOTU 828MkII, which eliminated the need for one.

That's where I'm a bit lost bitflipper - what's the best option if I want to use real faders rather than the virtual faders in Console View? I'm a slow learner.  ;)

The 1818VSL has lots of connectivity options for the Yamaha, SPDIF and ADAT. You have to purchase the ADAT card for the Yamaha and most digital mixers. Lot's of people using this with a DAW. 
http://groups.yahoo.com/neo/groups/Yamaha01V/info
 
Yamaha is not the best choice for a controller with Sonar, Yamaha's work better with Cubase, Pro tools and Logic. There was a patch for Windows XP but no one has bothered since. So I don't use it as a controller. But I do use mine as an Input source. I use it's DSP processing and output with SPDIF. Kinda like a 16 channel pre amp.  I also use it for playback monitoring control. But you sort of have this already with your audio box. 
 
The UA25EX will not connect to the Yamaha ( or possibly any digital mixer)  digitally as it has consumer grade TOS digital ports and no ADAT.  There is a converter available but a PITA. 
 
 

Thanks again. If I were to buy a V-Studio 700, do I also need to buy a Cakewalk VS-700R Audio interface or can it be used with a Presonus Audiobox1818VSL?

The VS-700 is its own audio interface. You don't need anything else. It's a nice unit.
 
However, if motorized faders were important to me I'd be more inclined to invest in a control surface with more than 8 of them. Unfortunately, that gets expensive.

Oh! But I want to use the Audiobox! It's got excellent reviews.
 
So . . . it seems what I need is a controller rather than a mixer, but as you say, 8 faders is of little or no use to me, I need at the very least 16 (even more!). Are there any with 16+ faders apart from the (very expensive!) SSL controllers?

There's the Mackies, expandable to any number of channels, 8 at a time. You're looking at a couple grand for 16 of them.