Make a note of the number then ring around any chipping firms to see if the number is theirs is the obvious answer but might be easier said than done. You could also ring your vets and ask them if they know which firm has the code to that number. Alternatively, if you have patience galore, you could try NED and see if they can help.
I contacted petlog and they came back telling me the new owner's have not changed the ownership from me :/ so i am now going to look at getting in touch with the passport people xx