Their HQ was stormed by police along with forced compliance of the destruction of information:
https://www.techdirt.com/2013/08/19/uk-govt-forces-guardian-...
https://www.theguardian.com/world/2014/feb/01/edward-snowden...
That said; in theory D-Notice's are voluntary, but non-compliance is vaguely threatened with "increased scrutiny".
D-Notices themselves are not exactly secret; they even get a mention in the movie V For Vendetta.