Version 4.0 of the MRR is effective from 29 September 2020.
SMETS1 Migration Reporting Regime V4.0 (clean)
SMETS1 Migration Reporting Regime - sample files - V3.0 (unchanged)
For reference, these are the prior published versions of the MRR.
Version 3.0 effective from 31 July 2020 - SMETS1 Migration Reporting Regime V3.0
Version 2.0 effective from 20 June 2019 SMETS1 Migration Reporting Regime V2.0