deducted breaks

[HPfan101]

Need advice on this issue, been working for my current employer since March this year on a zero hour basis did my 13 week trial but have received no contract where as in previous job prior to this I did. I'm not one for checking my wages, I know I should, so found out few weeks ago by a colleague that we get deducted 15 mins every 4 hours worked, I was shocked at this as I've never been told by any managers so never verbally agreed to or signed anything is this ok for my employers to do this?