My knowledge of tax stems as far as my wages go through PAYE
I remember when I was volunteering and I used to get cheques for my mileage (which I assumed that I didn't have to declare as an income as I wasn't being paid for work) and wondered if this 'bonus' would potentially be looked at in a similar manner. Instead of calling it a bonus, maybe a 'good will gesture' or something?
I bet I'm about to find out that I should have declared my mileage to HMRC