CGT is due on 15th December for gains made in the previous 1 January to 30 November (of the same year) and 31st January for gains made in the previous December. Don't get the payment date and the CGT filing date mixed up. Latter is 31st October following the end of the previous calendar year in respect of gains made in that year. Typically extended by two weeks for pay & file.
You must file your CGT retun on or before 31 October of the year that follows the date of disposal. You must do this even if no tax is due because of reliefs or allowable losses.