Sorry main thing i forget to state it has to be an automatic and achieve the 40-45 mpg on economy.

Why not spend a bit less, get a decent (and more reliable over the long term, as well as cheaper to insure and maintain) petrol auto, even if that (DSG/DCT aside) might 'only' give high 30s in mpg but doesn't break down expensively outside of the warranty period. The lower purchase and (aside from fuel) running costs would more than offset the lower mpg.

Realistically 20k mileage pa is still in petrol range, especially with all the reliability woes of diesels. The more reliable ones, such as the VAG 2.0 and Honda 1.6 will come at a significant price premium over an equivalent petrol auto, even after Dieselgate - its the unreliable ones with DPF and other expensive reliability woes that can be sourced more cheaply.

I own a petrol Mazda3, and I've noticed a BIG untick in the level of second hand diesel-powered cars on sale at Mazda dealerships in the last few years - as others have said, probably (in my opinion) because owners have experienced more issues than other makes and their petrol-engined cousins and so are off-loading them before something really expensive to fix goes outside of the warranty period.

In my view, too many prospective car buyers concentrate too much on mpg and VED for the petrol vs diesel choice, and not enough on the difference in purchase price (all other things being roughly equal), non-fuel running costs and the cost implications of reliability, especially if you intend to keep the car past its manufacturer's warranty period or a long time in general.

PS. Do you need the larger car because you are regularly hauling lots of kit around? Some C sector (Focus etc) cars (in hatch, MPV or estate) can be quite capacious boot-wise but use less jungle juice as they're smaller than the Mondeos, Octavias and Mazda6's of this world.

The same goes for all the 'good extras' - what do you actually NEED? No would like to have, but need. Something to impress the neighbour/colleagues/mates will become boring after a few weeks and may rarely be used, if ever. Unless you're buying new and money isn't a problem, I'd concentrate on buying engineering quality FIRST, then a comfortable drive (driving position, not too low profile tyres [preferably above 50] and below 18in - nicer riding and far cheaper to replace tyres) and optional extras come much further down the list.

Most people really only need a decent A/C and vent system plus a comfortable seat. The rest can be added on by you with sat nav and hands-free functions on your mobile phone (make sure its legal) for far less than done as an option on the car itself. Parking sensors are, admitedly worth having, but most cars built in the last 5 years have these as standard anyway, at least on the rear.

I'd do a budget of ALL the car's costs, buying an ongoing, including all maintenance (including likelihood and cost [find out from dealerships] of having to pay for certain repairs[including diesel forced DPF regens/cleans etc], based on evidence in the Reviews/...Make../Car/Good & Bad sections on this website), VED, insurance, likely depreciation as well as fuel costs. You'll be surprised at how well modern petrol-engined cars come out compared to diesels on below 25k annual mileage driving.

If only VAG offered their 1.4TSi with a TC auto box...oh well.

Mercedes C-class might be worth considering - the facelifted W204 version around 2011-2014 should be in budget. The diesel auto is reliable, the car comfortable. It might be a bit small in saloon version. Mine has done 42000 miles since new in 2014, and has averaged 55mpg, though that is mostly motorway travel. No problems at all.