HJ's realmpg is by far the better. Parker's algorithm seems to make no sense at all to me. Alter settings to make it more economical, and it comes out with worse predicted fuel economy.

As to the mpg ... it all depends. Which is a terrible answer. In general though, unless you're doing 15k + miles a year, petrol is better - less likely to get EGR/DPF problems, which can be horribly expensive, rapidly eating into any savings you'd made on fuel. But if your driving is all (or mostly) urban, then petrol probably keeps the edge for longer. A DPF needs decent runs at speed to do a proper passive regen, which urban trips don't provide. The active regens need similar conditions, and if interrrupted have been known to lead to all sorts of problems, from poor oil lubrication to runaway (read self-destructing) engines.

As someone else has said, you'll also get better economy on runs in the petrol focus. Whilst the figures won't match a diesel, when you add in the cost to change and the potential for problems, the savings won't be huge.

It may look like we're a bunch of diesel haters. We're not (or at least I hope we're not). I drive a BMW 325d estate, but the usage type it gets put to is suitable for a modern diesel.

All diesels from 2010/11 on, and a lot of diesels from before then (even back to 2003 or so), have DPFs fitted as standard. Remember, people generally initially bought those cars to do lots of miles and to save money on fuel. So they've mostly done big mileage, and things are going to be wearing out - scary time. If the cars haven't done much mileage, then it's scary time, because the DPF/EGR problems are more likely to suffer problems !

Your Focus is about the same age and mileage (mine's less at an 2005/55 and 52k miles) as my mk1 Mazda3, I have, like you, been thinking of possibly downsizing as an alternative to just replacing my car on a like-for-like basis, for similar reasons that mine can be a pain to park in tight road spaces with its high bootline (saloon) and no sensors to help me.

I have, however, conceded that as its a petrol (1.6) and has been thus far extremely reliable and hasn't (yet) rusted away, I may as well save on the inevitable depreciation buying a new or newer car/loss through selling my existing car and stick with my Mazda as its likely (again, as long as its not going rusty) to have up 100k miles left on it.

Like with my previous car (a 96N Nissan Micra), I will only change it if my circumstances demand it (e.g. if I took a job requiring me to do 20-25k+ miles pa) or the car suddenly had several and/or some very expensive repairs, especially associated with rusting/corrosion of structural parts, wherby they are soon to be terminal problems or cost more than the value of the car. The odd ding (especially on the plastic bumpers) on an older car makes less of a difference to the selling price than it being mechanically and structurally sound, so the odd slight parking bump (as long as its only your car that affected and doesn't require any proper repairs) will be incosequential when selling it say 5 years or so down the line.

If yours is still relatively rust free (especially regarding structural areas) and in reasonable mechanical order, then I would keep it going until the above criteria are met, then PX it/flog it if you can or scrap it and buy what you can afford at that time, which hopefully will be more than now if you save up for a bit longer. Your personal circumstances may also change at some point, so you might have to sell the smaller car and get a bigger one again, losing even more money if it were quite soon. You may also need to borrow money for other purposes that is far more important (a home, for example), and a car loan will make some difference to what you can borrow/afford if finances are generally tight.

I would keep the Focus and run it (not literally) into the ground. If you still have your heart set on a change, the mk2 Mazda2 is a fine little car (and 81% ave. on the real mpg across the entire range, which is actually quite good) if a little on the firm side in terms of ride quality; the Corsa and Polo by HJ's reviews aren't so good (especially in terms of reliability); the Fiesta is generally well-regarded (and share many parts with the Mazda2 if you're not talking about the current versions) and are probably the easiest to pick up a bargain, and besides, you may already have a good working relationship with your local Ford dealership, which can often mean just as much in the ownership experience when running second-hand cars (not necessarily for purchasing them, but for keeping them in good order afterwards with being ripped off).

Best of luck whatever you choose to do.

If buying first--then you will do better to sell privately rather than part exchange. Then keep the cash raised from the sale for maintenance or unscheduled maintenance on the Mazda.

All in all tho'-if it was me-I would still be inclined to keep the Focus for a year or three-but SAVE for a Mazda ....that's cheaper than a loan for a cheapo cheapo second hand car....and far less hassle into the bargain.

If there is the possibility of your moving to a company car in six months--with that car intending to be your sole vehicle--then there is absolutely no reason at all to do anything other than "think options" at the moment. Stick with what you've got till then.