All they have to do is put up a sign outside their mosque:
"All Moslems have a religious duty to assist the authorities in apprehending and convicting the 7/7/05 terrorist bombers."
And say the same thing in their sermons, in both English and Arabic.
Most Muslims don't know Arabic--they read translations. In England, the sign would have to be in English and Urdu. Like our "immigrants" from south of the border, many Pakistanis and Bangladeshis who live in England don't bother to learn the language.