bought some second hand seat belts to replace some chewed ones in my van they worked fine when removed from the old van but now they wont work at all the van was a breaker no damage so the explosive bits are still intact just cant get them to extend out any more

You need to remove them, and turn the 'retractors' around until they 'unlock'. Then make sure you don't turn them upside down again before fitting. Its to do with how sensitive the inertia locking latch on them is. Had the same issue on a mk3 Panda's rear belts