After completing the UKMT Maths challenge, you receive a certificate of completion and a key fob.
The key fob always contains a puzzle on it. See this site for previous years. This year it was a cipher:
IMOK 2017 Decode: 2 50 17. 0339 03 28 92046796
There is also the hint:
'It may help to think about some of the obvious letters. For example, there are not many choices for “2‘.'
Can anyone decode this?