Contact details for NS+I who issue all Premium bonds and a range of savings products.
www.nsandi.com/how-do-i-get-touch
You'll need to give them quite a bit of information for accounts that you've held as a child/still hold as an adult and it will have to be in writing. Other options are there for other enquiries.
They will be able to tell you what's won and when it was.
From that link "Write to us at NS&I, Sunderland SR43 2SB
The more details you can give when you write to us, the more quickly we can deal with your request:
Your personal details: name; address; date of birth. If you haven’t told us about a house move or a change in your marital status or name, please also include your previous name and address.
Your reference numbers. Any or all of these: your NS&I number, holder’s or customer number, account number, Bond numbers, or Certificate numbers, if you know them. You’ll find these numbers on your Bond records, certificates of investment or any other letters from us.
Details of your accounts or investments: type of investment (eg Premium Bonds); approximate start date; and amount, if you know them.
Don't forget to sign your letter. For your security, we can’t disclose or change personal information without the account holder’s signature(s). If you are over 16 and the investment was made for you when you were a child, we might not have your signature on our records. In that case, your signature should be witnessed so we can transfer control of any investments to you. The witness must be aged 18 or over and must not be a member of your family. The witness must also sign the form."