Re: Employment tribunals
If it wasn't formally laid down then the leave year starts on the date you started your job
From Gov web site:
Leave year
An employer must usually tell their staff the dates of their statutory leave year as soon as they start working, for example, it might run from 1 January to 31 December.
Workers must take their statutory leave during this time. If a leave year isn’t set out in a contract then it will start:
on the first day of a new job (if started after 1 October 1998)on 1 October (if started on or before 1 October 1998)
The leave year and holiday entitlement is not affected by maternity, paternity or adoption leave. The employee still builds up (‘accrues’) holiday over these periods.