I recently helped out a client with an HMRC matter. The final result was significantly better than the client was expecting, and they were so pleased they said they'd like to give me a bonus. It was genuinely non-contractual and unexpected, and was the first (and I'm sure the last) time it's ever happened.
I then went to get my hair cut and told my hairdresser, who is also a client of mine. We both were impressed at my importance and new found wealth, although ironically when I went to pay my debit card was declined due to lack of funds. How we all laughed. Well, when I say "we all", I mean mainly my hairdresser and the other clients in the salon.
Anyway, sorry, back to the serious point. Is this bonus taxable? I must confess I was keen for the client not to "forget" his kind offer, so I sent him an invoice that included the bonus amount (plus VAT... that'll teach him).