https://www.gov.uk/government/publications/private-trusts-of-land/practice-guide-24-private-trusts-of-land

The Land Registry provides a helpful Practice Guide on registration matters relating to Private Trusts of Land. This also explains which forms should be used to register the severance of an equitable joint tenancy. It also offers a helpful YouTube video which explains how to fill in a TR1 and what the various entries mean: https://www.youtube.com/watch?v=nIpPHDUAae4. It is very clear on the implications of joint tenancy and tenancy in common and how to complete the relevant section of the TR1 form.