GOVERNORS at Britain’s top terror jail believe al-Qaeda extremists are planning a major attack as payback for Osama bin Laden’s death. Security has been stepped up to “unprecedented” levels at Belmarsh prison after warnings that the Muslim Boys gang is plotting a revenge mission. A 15-strong riot squad in stab-proof vests has hidden around the jail’s chapel during Muslim prayers on Fridays in case of an attack... Prisoners on House Block One, which includes 39 terror suspects, added to the tension by staging a mass show of defiance on Friday. Inmates refused to return to their cells and a source...