In reply to Tall Clare:
> Audi A6s tend to have curved backs, which reduces load capacity when you want to put a dog cage in.
We have an A6 Avant and the boot is quite capacious, both wide and long but negating that is the fact that the boot floor is high as Audi have left room for a full-sized spare wheel (a great thing to have in this day and age!) as well as room underneath for a rear differential needed for the 4wd quattro versions. As you rightly say, you'd struggle to get a big dog cage in the boot. Despite being big, there's still a significant element of 'lifestyle' styling in the A6's design, rather than out-and-out practicality (perhaps unsurprisingly considering it's an Audi).
I've read that the Ford Mondeo estate has more room, seats up or down, than any other estate car currently on sale in the UK. A Ford would certainly be cheaper to maintain (and insure?) than a Volvo but it'd also depreciate more, so it's 6 of one and half dozen of the other ... Owning a Volvo over a Ford may give more satisfaction/ kudos/ image etc, but you'd definitely pay the extra for it. An Audi, Mercedes or BMW would depreciate less than a Volvo - hence the higher mileages or older cars at £10K - but if they aren't fit for your purpose then I agree, they can all be set aside.
As already advised, drive both and see which one suits your needs the best, which one's the nicest to drive, most comfortable to travel in etc. I strongly suspect that the Mondeo would be the best handling car and also has the wider choice of power outputs - as far as diesel engines are concerned at least. (In contrast, our principal needs were for a big diesel estate car with permanent 4wd and a proper spare wheel - so, perhaps unsurprisingly, we ended up with a quattro avant.)
Good luck with your search.
Dave