If you have very simple tax affairs the P800 computation may be correct, but in other cases there may be changes required. In a few cases the P800s have been issued to taxpayers that are already registered under self-assessment, and do not reflect the entries already made in their tax return.
James McBrearty specialises in making tax easy for the self-employed. He founded taxhelp.uk.com in 2006 and is also the author of 2 books for small business. In 2016 he launched an online business course that now has over 3,500 students in 124 countries around the world.