I'm aware of most if not all DLA receipients receiving an extra tenner around xmas, has happened for a number of years as far as I know.
I'm being pedantic, but is 'bonus' really the correct term for them to be using?
Formal complaint to 1st Credit followed by a complaint to the Financial Ombudsman service, if you're unhappy with 1st Credit's final response to your complaint. It will cost 1st Credit a case management fee of £500 (I think) if you do refer to FOS.