In general the tariff applicable when they applied for preliminary accreditation, but with one exception.

This from the Phase 2B review:

Once accredited, installations found to be eligible for preliminary accreditation, will receive the tariff that they would have received if they had accredited at the time they applied for preliminary accreditation. However, installations that are granted preliminary accreditation with an effective date in the period 1 January to 31 March each year will be eligible for the tariff that applies from the following April. Tariff lifetimes will apply from the eligibility date.