I reckon it is covered by s13 of the Employment Rights Act - you can read it here, if need be. This is about Rights not to suffer unauthorised Deductions.
http://www.opsi.gov.u...18_en_3#pt2-pb1-l1g13
The remedies under this Act are contained within s23 - Complaints to Tribunals - so talk to the ET people now. You are right about the 3 months limit for making a claim.
All of the points that factor makes are true.
Also, if your last pay week was 2nd August (a Sunday, so your last day was Friday 7th), check that they haven't deducted NI on your holiday pay if they chose to put that through in the following week's payroll run - its more than likely the system would deduct NI automatically. You do have to pay income tax on holiday pay, but you only pay NI for the weeks you worked - i.e. until 7th August.
You can download the ET Forms here. Good luck.
http://www.employment...nce/formsGuidance.htm