Re
I would push it as far as possible Machine with the company, the clause is fairly explicit , ask them to explain in writing why they cannot honor the bonus in full and then tell them that you will need to speak with a third party for clarification on the matter by presenting your contract and their letter of explanation.
Then leave it a few days and do just that ,from memory i know the labour commission do have a legal advisory section , but ultimately they will refer you a solicitor.
If i was you i wold also let them know that you have "banked" on this bonus since you passed you objectives and to rescind the money now will leave you in a very tricky situation personally , use the owl emotional lever also !!....Someone in there will have to make the call so just let them know that if they rescind on the clause , it will be them who will be responsible and answerable.
Me thinks they are just testing your metal