The BBC story pretty much says it all:

Scotland’s top prosecutor has said the case has not been made for extending the length of time terror suspects can be detained without charge to 42 days.

BBC Scotland has learnt that Lord Advocate Elish Angiolini gave her opinion in a letter to the Liberal Democrat MP Alistair Carmichael.

She said the change from the current 28 days was not supported by “prosecution experience to date”.