I had a bonus in December of £180 and they took all of it not just 70%. Thought it was a bit harsh at the time, I was expecting to keep a little bit lol.
As I understand it, the policy is the same - a cash bonus is classed as part of salary, but the proportion claimed will depend to some extent on individual circumstances and the amount of the bonus. As far as I can see there is no hard and fast rule that says if the bonus is x then the amount claimed will be y