My 2007 Focus 2.0 TDCi Titanium came fitted with the black Sony 6CD Changer (not sure on the model number) (this one: http://www.caraudio-service.ru/Ford_CD132-MP3-CDS_3S7T-18C939-AD_Sony_CDX-FS132.jpg)

Is there a Ford unit which includes Sat Nav which is compatible with my car? Or will I have to get an aftermarket unit?

You may as well get an aftermarket Satnav as you will not have the center display (large clock display) so you won't benefit with the ford model, depending on how much you want to spend there are lots out the to choose from , but you won't get the same sound quality as the sony unless you move to maybe the pioneer model which is just not worth it. They start from £250 on Ebay and certainly look the part as for if they are any good i don't know.

To change to the oval style you can get surrounds to modify or you can get the square shape to fit your dash.

Maybe being optimistic but is it simply a matter of taking the Sony out and putting a replacement in or is there likely to be more than this involved? Can you recommend any suppliers? Just so I can have a look at some units.

If you go on Fleabay and type " Ford Satnav" it will show you a good selection of satnavs from £250 upwards , most come with the required interface for a direct change. So most would be a straight swap unless you go for the oval type which you would need a new surround to fit from the square shape.

The ones on Flea Bay look like cheap Hong Kong versions. Do companies such as Pioneer, Sony, etc make aftermarket devices to fit Fords?

Yes Pioneer do , but IMHO they are not worth the price , i don't think Sony do , search the Pioneer one on Ebay it will come up , they are alot of money when you can get a cheap Tomtom , mine came with Satnav , until then i used my Phone.