It might not be blocked, blocked, but it could be a warning. This is the Corsa C forum (2000-06) and it sounds like you have a later car. If it's still running normally, you could try a good blat down a fast road for a good while. Diesels with a dpf don't like lots of short, stop-start journies so a good run at 50mph or more and allow the revs to keep higher than you would normally in lower gears will help to get temperatures high enough to get the soot burnt off and blown out. It might be worth putting a dpf additive (about £14) in with the diesel before the run.