Legal Challenge

Apparently Bercow can sign the letter in the PM's place.According to Joanna Cherry if Boris doesn't comply by 11pm that is what will happen

EV I've also thought that there has been a miscarriage of justice in that case. It did seem at the time that the police had found the person they thought responsible and didn't follow any other avenues.

Re Brexit I've read that the Scottish Court can write to the EU requesting an extension.

I wouldn't hold your breath at getting an answer.
A case that I've followed for 34 years which involves a man being in prison for 33 years having been charged with a murder he didn't commit ( front page news in todays Mirror ) umpteen requests have been sent to the Home Office for release of withheld information from Essex police, but to no avail . The police have held on to vital information which should have been used in the man's defence but for reasons only they know, they're not releasing anything.

Even with the use of court orders to EP to hand over files/documents has fallen on deaf ears so no matter what method of contact is used it's completely ignored.