Hi Rowley,
I don't have a diesel x-type but my previous two cars were a Mini Cooper D and an Alfa Romeo Mito 1.6JTDM. The mini didn't suffer any DPF issues but the Alfa's DPF light used to come on regularly. The advice given to me by an Alfa specialist when the light came on was to, "drive like an angry Italian!"
I was talking to the guys at Jag Auto's in Liverpool last week and in passing they said that they recommend all diesel owners take a ten minute motorway trip in 4th gear every 4 or 5 weeks to clean the filter out.
Hope that's a bit of use to you,
Ian