This article  from Exaro in January 2013 (ie 12 months ago) shows that there were at least 30 active Police Operations into child sexual abuse by groups. The figure would be higher as several forces did not reply.
Resources on child abuse investigations are now similar to those used on murders and serious organised crime investigations according to a further article 
Last year the Exaro site, who often specialise in journalism on child sexual abuse was paid subscription only but now it is open and free to all.
They often work with the Sunday People and Mirror as with this short report  in the Sunday People today which says undercover police pose as Sunday People journalists.
cathyfox The truth will out