Can't get key to turn properly?

Hi All,
I have a problem fillin gthe boiler, it's a Worcester28CDi. I have put the key into the black pipe lined up to the unpacdlock picture and then turn to the lock position but it doesnt lock. If I then turn the grey tap to fill up the boiler then water leaks out from the key (black pipe) area. I have been trying all dy to get the key to lock but it simply won't. Any ideas?
thx