You asked

I understand that ONS has a "bespoke Memorandum of Understanding" with Ordnance Survey for the purposes of using OS data, including data from OS's AddressBase product, in the preparation of the ONS UPRN Directory (ONSUD) and the National Statistics UPRN Lookup (NSUL). This MoU is in addition to ONS's participation in the Public Sector Mapping Agreement.

I would be grateful for a copy of the MoU. Please treat this as an information request under the Freedom of Information Act.

We said

We have a strong relationship with Ordnance Survey (OS), working together to maintain a number of national reference products, including boundary, postcode and address data products shared via the ONS open-data portal.  

The relationship is outlined in an MOU with OS, published in full on our website;

https://www.ons.gov.uk/news/statementsandletters/anambitiouspartnershipprogrammebetweenonsandosmemorandumofunderstanding

This MOU is currently under review. The revised version will be published in full on the ONS website when completed (expected early 2020.

The information you were given before wasn't quite right as a bespoke MOU is not required.

We have applied for exemptions under the normal terms of the Public Sector Mapping Agreement for creation of its postcode products and these have been granted:

https://www.ordnancesurvey.co.uk/business-government/licensing-agreements/public-sector-mapping-agreement

We apologise for getting this slightly wrong before.