Hi Steve,
 
   I just purchased the Saffire PRO 24 (the Scarlet interfaces are the USB driven equivalents) recently based on feedback from this post. There are a lot of options for an audio interface, so research what suits your needs. In general, you will want one with:

1. ASIO drivers
2. Multiple inputs (XLR, Line-In, MIDI in/out)
3. Phantom Power (to drive condenser mics)
4. Either USB or FireWire connection to your computer

   The interface will need the USB or FW to your computer, and use can use your mixer to connect to the inputs of the interface.
 
   There is also a trick you need to know with Win7 to make it not grab your interface when it sees it turn on (this thread may be helpful in its entirety as well). That will let you let MC6 see your interface in ASIO (exclusive) mode for the best latencies.
 
   In all honesty, with a good interface, you may find your mixer to be unnecessary, as you can record into the computer and let MC6 be your new "mixer."

Now days, I would recommend a USB connection. Fire wire can be a bit problematic if the FW chips are not 100% compatible. I've not had this issue but I hear that some folks have had problems.
 
Yes, the interface becomes the sound card for input and output.  With the right interface, no mixer is needed.  However, should you opt to use the mixer,   that is totally fine. Yes, send it's output to the interface. Set levels accordingly.  You can use the mixer for monitoring but I think you need to be careful because that could be a potential feedback loop. I recall this issue being discussed at length many years back by several here who were using the very setup you are describing.
 
My setup, as I mentioned, is a bit simpler. The interface is the common point and no mixer is used for anything going in.
I send the output of the interface directly to my studio monitors. The interface keeps the inputs totally separated from the outputs so no feedback can occur. I only have to be aware to turn the monitors off when the mics are hot.
 
My outputs echo my inputs and the playback so I have full duplex available at all times for sync recording. What I hear in the outputs is exactly what's happening in the tracks and the mix.
 
Have a listen to some of the music on my webpage that I have recorded using this simple setup. You might wish to consider using it as well.
 
When I first was getting started, I wanted to have a really "cool looking" studio..... you know, with the mixing desk full of faders and knobs and flashing lights. I was planning to use my D-1600 in that way. I had very little available desk space, and after setting it up, I was having a slightly distorted sound. That's when I decided to try the mic straight in and it was crystal. The D-1600 went on E-Bay the very next day.
 
I don't care what the studio looks like... to me, the important part is what sound quality I can get with the music. I've had a few singers come over to record some stuff, and I see a bit of disappointment in their eyes when they don't see what they are expecting to see..... but that changes when they hear what my humble studio can do.

Thanks for your help and advice. I'll keep the outboard stuff for now but will look at getting something like a Focusrite Scarlett ? There is no midi connection with this but presumably my UM-1 can still be used to connect my non usb midi keyboard without causing any problems?

The UM-1 should work OK as in input device for the midi keyboard and with the Scarlett using ASIO and handling the audio signals in and out..... you should be OK.  Lots of keyboards these days don't have the 5 pin din connector anymore and come with a USB port built in so that would effectively work the same way....
 
Let us know how it goes.

This may not be the right place to ask but I'm thinking about getting the Focusrite Saffire Pro 14 interface. My firewire card has a VIA chipset (not sure which one). VIA are on the Focusrite recommended list so will I be OK? From what I have read Firewire has some pro's over usb.

While many FireWire device manufacturers recommend TI FW cards, I have a VIA in my music computer and never had any problems with it and my Saffire.
I think if the VIA is on Focusrite's 'good' list, you'll be safe and will (probably) get support if there are any issues.
 

I also had the same concern when I purchased my Saffire Pro 24 (as I have a VIA chipset as well). I have not had any issues with the PRO 24 at all.
 
Because I had been spooked some with the "be sure it is TI" comments, I did post a link to a TI card in my #11 thread above just in case, but did not need to use it.

Thanks guys. I will go for it!