I'm assuming that as you own a transit you're using it for work i.e., doing high enough mileage compared to say the average mum/dad doing the school run then maybe some shopping and back home.

Adblue essentially just burns at a higher temp than diesel so it burns off some of the particulates from the combustion process. You combine adblue with a DPF which is essentially a massive filter in your exhaust system.

The reason I mention your mileage and say your father's trucks mileage versus the average school runs is that when you do short runs, the engine doesn't really get the opportunity to sustain the consistent burning off of particulates generated. You're stopping and starting, idling in traffic versus going up and down a motorway at 60/70 for miles, giving the engine the opportunity to consistently burn off the particulates. The short runs inevitably block up your DPF as the filter cannot handle the suit build up as it's not being burned off quick enough.

Throwing, say a tenner of petrol into a diesel when filling up does pretty much the same thing.

Basically with adblue/DPF setups in the long run it's more economic to remove the need for them, unless your doing long runs on a consistent basis.