buy a new one

when you refit the chain over everything (there are coppered links there so you can't go wrong with the timing) make sure the "bomb" hasn't gone off by wedging a screwdriver in the way
once everything is set up correctly, remove the screwdriver and turn the engine over by the main crank pulley bolt
this will release the plunger and everything will be fine

don't worry about the markslining up again ebcause they won't, but you will need to wrry about it if you have timed it up wrong because you'll need another new bomb at £25 a time

i'll post up the whole timing thing if you like