The Code of Conduct describes the responsibilities that MPs have as Members of the House and are based on a set of principles. 

The new Code of Conduct and Guide to the Rules relating to the conduct of
Members took effect from 1 March 2023.

You can see a copy of the letter from the Parliamentary Commissioner for Standards here: https://www.parliament.uk/globalassets/documents/pcfs/talks-and-materials/2023.02.27-pcs-letter-to-mps.pdf

You can find a summary of the changes here: https://www.parliament.uk/mps-lords-and-offices/standards-and-financial-interests/parliamentary-commissioner-for-standards/code-of-conduct-and-rules-of-the-house/

and the text from a presentation here: https://www.parliament.uk/mps-lords-and-offices/standards-and-financial-interests/parliamentary-commissioner-for-standards/parliamentary-commissioner-for-standards/talks-and-materials/changes-in-the-new-code-of-conduct/

The full Code of Conduct can be found here: https://publications.parliament.uk/pa/cm5803/cmcode/1083/1083.pdf

All Members and their staff are urged to familiarise themselves with this document.