I was looking at a Focus but my attention's been drawn to a Mondeo Titanium X, is there any reason not to go for an Eco-boost Powershift model?

A BMW 320d should be about the same as a 2.0TDCI
A BMW 330i (Petrol) is supposed to do 39 MPG
very nice, but after deisel. Didn't realise those were petrol.
Man up and tell your wife to pipe down and buy something that you want.
Skoda Superb 2.0 TDI PD 140 SE 5DR
Petrol offers better VFM. Will take you a while to recoup the difference in price between diesel purchase and petrol purchase...