The Department for Transport (DfT) have published Authorisation 836.
It permits the continued use of the tanks specified that the material thickness is below the minimum required in ADR but above the minimum required in the Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations.
It remains valid until such time that the tank in question does not possess a current inspection certificate, unless otherwise revoked.
Authorisation 839 can be found on the GOV.UK website.
Last reviewed 4 October 2019