Hello Alistair, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
1/3 acre is below 0.5 hectare, the permitted area. Thus any plot disposal attracts Private Residence Relief (PRR) which relieves Capital Gains Tax (CGT) at 100%. There remains one danger, however. HMRC may regard the transaction as 'trading in land' and subject any profit to Income Tax (IT) as opposed to CGT.
I do hope that you have found my reply of assistance.
Thank you for your support.