Sources
The official sources your agent can search and read through Solved.
Legislation
UK primary and secondary legislation, such as Acts and statutory instruments, published by The National Archives on legislation.gov.uk.
Your agent can read legislation as it stood on an earlier date, where that version is available.
HMRC guidance
HMRC's internal manuals, published by HMRC on GOV.UK.
Your agent reads the current version of each manual section.