how can i fix my handbrake lever?

i have a 1999 vauxhall frontera 2.2 dti limited, the button to release the handbrake keeps falling out, when it falls out the handbrake lever will not lock into place, i can push the button back in but the next time i use it it just falls back out again, are there any parts that i can replace or will i have to replace the whole lever?

p trehearn
September 2008