Was up early feeding DS and was doing some internet window shopping, decided to see how much money was in my account, I knew there should have been around £18. And yes, indeed there was the £18 I thought, plus £300 
Sometimes my grandma will transfer me money but when I log into my online bank, my grandmas transfers say nothing, they look like this-
123456 / 12345678 £123
So her sort code and account number, then the amount she transferred to me. This random £300 however, looks like this-
Bank credit FRM
12345678901234 £300
The wording, where grandmas don't say anything, what does FRM mean(?) plus the one really long (14 digit) number) that don't match my grandmas numbers. It can't be a cash deposit because apparently you can't deposit cash into other peoples accounts anymore, something to do with money laundering laws. I do get child benefit but the payments look like this-
Bank credit 123456789012-CHB £123
Tax credits go into a different account and no-one else has my bank details to pay me any money
So where on earth is it from? I hope it's not a mistake and that it's legit but that possibility seems slim
I will ring my grandma later on today but if it's not her I've no idea!