Hi Matthew,
Thank you for your question. I believe you are referring to the remittance basis found on help sheet HS264
Paying tax on the remittance basis (Self Assessment helpsheet HS264), which outlines the basic principles of the remittance basis. Without sight of both Tax Returns, I would be unable to advise in full as other circumstances have to be taken into consideration. I would advise to contact the Self-Assessment helpline on 0300 200 3310 as this forum offers general advice and is not catered to individual circumstances.
Thank you.