DPFs are on diesels, your 3 is petrol isn't it?
They filter the smoke, which is a good thing. The problem is with how they are cleaned out. There are two kinds (on cars). The VW kind and I think maybe Peugot etc are cleaned off by the engine increasing the exhaust gas temperature by various means; the Ford kind are cleaned out by means of a chemical that you have to add, and I suspect some people forget to do. The more long motorway or fast trips you do, the less it needs to clean it out by these other means because the hot exhaust gas will burn it all off anyway.
If you only tootle round town (on a VW at least) the filter gets a bit blocked up and a light comes on. The manual tells you that when the light comes on you have to go and drive for 30 minutes at over 3krpm or something, but of course a good sound thrashing of any kind will have the same effect. I also suspect some people ignore the light..
Some people are saying Ford ones always need replacing after 70k miles at a cost of £700, I find this hard to believe....
Yes the 3 is petrol. Will probably be getting a diesel next though so interested in knowing about this fault.
[i]My partner has the option of leasing a car through her work[/i]
Just cos she can, doesn't mean she should.
She needs to do the sums and decide whether she wants to pay for a new car vs s/h car and (probably lots of) spare cash.
No dpf on the 1.8 Tdci which is the ford engine. I would go for a Honda Jazz 1.4 petrol. Great little/big cars!