Take it to your dealer. It may be a stress crack. Some MINI's have had problems over the years with windscreens cracking for no reason. It's usually at the top left hand side of the screen.
If the dealer agrees, you will get it replaced for free.
By the way, Autoglass are good, They have replaced a screen (standard screen by the way) in mine after a rock hit it off a lorry on the motorway. Did a good job. They fitted it at work using a mobile unit.
The only downside is you loose the MINI logo off the screen, but mine was the same make as the original one though.