When to pay CGT?

I sold some shares this week & the proceeds are in Sterling. Does anyone know when I have to pay the CGT on the profit and can I use the value that actually reaches my bank account in Euros? Or is there some official exchange rate that I need to use?
I file a tax return each year in October, but I think the CGT needs to be paid sooner.

For Gains between Jan 1st and 30th November, you must pay any liability by 15th December.

Thanks Gervan.