If you're given too much change, do you have to give it back when you realise?

I ordered a takeaway tonight and my meal was £12 something and I paid with £20 and got £16 back...didn't think cause it was late and I was tired, only got a takeaway cause my fridge broke and didn't realise, didn't get my meal till 11 so not concentrating...so a bit later the doorbell went. Still hadn't realised the extra change that I put straight in my pocket...of course at that time of night, I'm not gonna go answer the door if I'm not expecting anyone. I just realised the extra money I was given and now I realise it must have been the driver coming back to get the extra money. Do I need to call the place and let them know? I don't mean morally, I mean legally. I have savings so for my prison boyfriend for xmas and money is important for him so moral doesn't mean much to me when it comes to taking care of my man and it's also not my fault if they're as stupid as I am not to notice the extra money. So I only want to know if legally i have to give it back.