holiday pay

[lofty4200]

i am working for a agency as a hgv driver and have done so for the last twelve years for the same company recentley jobs have not been available as in the past so i am working sometimes 2 days other times a full week can anybody tell me what my holiday entitlement is i was told 12 weks previous to takiing holiays was the average pay my boss is saying 12.5 days is all i get what is correct