Despairing of finding a logical reason for this detention with no charge for 42 days malarkey (well, a logical reason other than 'Gordon wants it') I went to the ever-reliable (except when it's not) Wikipedia and it all became clear: 42 is the obvious number to choose!
(Italicised quotes are from Wikipedia)
In Mathematics it's the second sphenic number to be bracketed by twin primes.
In Science, it's the atomic number of molybdenum.
It's the angle for which a rainbow appears.
According to the History Channel. If one were to create a tunnel through the earth connecting two cities the gravity propelled trip would take 42 minutes regardless of the choice of cities (about the same time as a trip down the lower colon of Gordon Brown).
In astronomy Messier object M42, is a magnitude 5.0 diffuse nebula in the constellation Orion, also known as the Orion Nebula .
In religion the number 42 appears many times in various tracts. In the Bible, there are 42 generations (names) in the Gospel of Matthew's version of the Genealogy of Jesus; it is prophesied that for 42 months the Beast will hold dominion over the Earth (Revelation 13:5); 42 men of Beth-azmaveth were counted in the census of men of Israel upon return from exile (Ezra 2:24); God sent bears to maul 42 of the youths who mock Elisha for his baldness (2 Kings 2:23), etc.
In literature, Carroll's Alice, has 42 illustrations. In Chapter XII, the king explains "the oldest rule in the book": "Rule Forty-two. All persons more than a mile high to leave the court". Carroll also uses the number in a line in The Hunting of the Snark : "He had forty-two boxes, all carefully packed..."
There are the Kumars at 42, the episode of Doctor Who called 42 which lasts 42 minutes....
There is the rock group Level 42 and the Icelandic Band Strigaskór nr. 42.
In sport there are 42 laws of cricket. It was the jersey number of Jackie Robinson, which is the first and only number retired by all Major League Baseball teams.
In technology, 42 is a common magic number used by programmers. In the TIFF image file format, the second 16-bit word of every file is 42, which is used together with the first word to indicate byte order. In the reiser4 file system, 42 is the inode number of the root directory.
And of course there was a book, show and film written about 42nd Street.
....and most of these references pre-date Douglas Adams and 42 - "the answer to Life, the Universe and Everything".
So it is perfectly obvious why Gordon Brown has chosen 42. Oh ye of little faith!
Gordie might however, ponder on the reason that Douglas Adams gave for picking 42:
The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk, stared into the garden and thought '42 will do.' I typed it out. End of story.
I suspect Gordon Brown will give much the same explanation, if he is honest, when he comes to finally writing his memoirs and writing up this "42" episode.