It has not changed in the contract you signed on joining. As far as I know, there is no agreement on Holiday/leave entitlement beyond resourcing and any changes in employment law.
From my contract it was.
Holiday Entitlement
Your holiday entitlement will be:
On Entry 4 Weeks 2.5 Days
After 5 Years 4 Weeks 3.5 Days
After 15 Years 5 Weeks 1.0 Day
After 20 Years 6 Weeks 0.0 Days
If your hours are part time this entitlement would be pro-rata.