Apprenticeship rights helpWatch
Quick guides to apprenticeships
Sounds like he's writing the £30 a day off as expenses or something to avoid tax.
Does it make a difference to the college whether you're getting paid or not?
My contract states my pay in it. It shouldn't be a problem for him unless he's trying to hide it from someone. If you're not comfortable with questioning him about it (I know I wouldn't be) then tell the college and they should speak to him.
If this slides he could take advantage and not pay you at all 'because that's what it says in the contract' that you will have to sign and agree to.