If you post in customer service with as many details as you can such as the rough date, it might help to have the serial number of the board handy (I'm not sure if OCUK do it, but some companies log serial numbers as part of the sale), they may be able to help you.
If you gave your email address or store account details I imagine it would be relatively simple, but it's been ten years since I bought from them in person so I can't remember if they linked the purchase to my store account.
If you're not sure of the exact date, but you bought ram or a cpu at around the same time time and have the receipt for that, it might help narrow the date range which i'd imagine would help them find it.
Basically they'll have a record of it, but I've no idea how easy it will be to find for the CS staff if it's a true cash purchase so every bit of info you can give would help.