A chargeable event & tax

My mother died earlier this year, leaving a life insurance policy which she had taken out 11 years ago. It named me, my brother and her as lives insured. My brother and I have now cashed in the policy, which has given us a sizeable chunk of cash as a 'chargeable event' for tax purposes. The combination of the gain and my normal income hasn't brought me to the higher tax rate threshold - at least I don't think it has, although I'll need to check the interest on my savings.
Because my income is usually PAYE only and I have nothing to claim for, I don't usually fill in a tax return. I'm wondering, though, if I should this year in view of the 'chargeable event'. And if I should, when does it need to be done? The gain was made in September this year.