Yeah its a fairly common problem on most newer diesels tbh, as it should be replaced around 100k or so. The best thing to do is buy a Mk4 Golf TDi or Vectra C CDTi to avoid this as they have no DPF.... 
Back on topic, I'd also go with lambda at fault, easy to check with a multi-meter...be best to get it sorted sooner rather than later as if it causes over-fuelling that could kill the cat.
Back on topic, I'd also go with lambda at fault, easy to check with a multi-meter...be best to get it sorted sooner rather than later as if it causes over-fuelling that could kill the cat.