If you only use it for short journeys you should give the car a long trip to blow stuff out every do often. This helps catalytic converters out as well as DPFs.
I commute 15 miles each day so I don't have that problem but all cars need to be run to proper temps just to keep them healthy.
Same here, I do 15 miles each way, most of it motorway so I've never had a problem with the DPF. I got documentation with the car telling me that it should have a decent 20 minute run each day to keep it ticketiboo as it were.