It's a bonus, so legally quite within their rights not to pay out if you don't meet the requirements set out. Most companies normally send out bonus requirements before they pay them to you in writing.
Quite often find therre is always a requirement in there with regards to attendance. I know...
Used to get a extra £100 in our December pay packet. Now we get 15% staff discount instead of 10% for four weeks over Christmas and New Year. *******s! Yo do the maths meaning we would have to spend a fortune to get the extra £100 back, plus it means they get all my wages back in their till...