First things first; the above answer by tamborine is completely wrong. There is no differentiation between part-time workers and full-time workers in regard to holiday entitlement. Overtime may be differential, but it would have to say so in the terms of the contract (what typically happens is that some part-time workers are not permitted to have overtime rates until their number of hours worked in a week exceeds the contracted hours of a full-time worker).
So, as beeen said on here many times before by Buenchico and others, you get a minimum of 5.6x your 'contracted hours in holiday. This may include public holidays. The 5.6x is a legal minimum.
The issue for you, as others identified before tamborine 'butted in', is 'what are your contracted hours'. Within 2 months of starting work, your employer should have given you a principal statement in writing of employment particulars. This is also a legal obligation, and if you had it now, this issue wouldn't apply because one of the things it must tell you is your hours of work (from which you can work out the number of hours). Ask for this now.
This business about increasing your contracted hours 'when they can afford it' is tripe. I suspect they are going to get away with this holiday hours issue now, because you can't prove it one way or the other until you get the Written Statement.
http://www.direct.gov.uk/en/Employment/Employe es/EmploymentContractsAndConditions/DG_175704