Meal allowance payments

Our lot or depot,I’m not sure which have a real bee in there bonnet about drivers doing 6/7 hrs per shift/30 hrs p.w and having to pay them 50 hrs( you can only do what work is put before you) garaunteed p.w,they tried to come up with x / y why they didn’t have to pay you 50 hrs,but contract was shown to them so they have to pay it.
They now seem to have given up on that one and have turned there attention to our meal allowance which is £15 per day/shift,there saying( memo on board) that if you don’t do 8.5 hrs per day/shift there not paying it.
My contract says
a meal allowance of xxxxxxxx will be paid for each full shift you work
No mention of hours/ length of shift etc,just that sentence,so how do we stand :question: ta