The issue of unregistered Muslim Marriages is something I have Blogged on in the past see http://religionlaw.blogspot.co.uk/2010/02/muslim-marriages-4.html where I give links to my other Blogs on this subject.
I have consistently said that in my view those who perform unregistered Muslim Marriages should be prosecuted under s75 of the Marriage Act 1949 which makes it a criminal offence to solemnise a Marriage in England and Wales otherwise than in accordance with the Act.
There is a similar, though rather more straightforwardly worded provision in s101 of the Australian Marriage Act 1961.
In Australia it has been reported that an Imam is to face trial for conducting an unregistered Marriage though in that case the alleged criminality is aggravated by the fact that the "Bride" is reported to have been 12 at the time.
It will be interesting to watch this case and see if it might inspire the English Police to start to enforce our Marriage Act in the same way as the Australian Police are enforcing theirs