[aka23]

The return/variance calculator above doesn't evaluate risk of ruin. I will have a return/variance simulator that does do risk of ruin later this month.

Assuming the bonus is given pre-wager. You have a ~0.02% of busting with £1 bets. With 5 and 10 unit bets, the risk increases to ~9% and ~22% respectively. To check others, there are risk of ruin tables on the bottom of the blackjack page on my site and in one of the blackjack appendixes on wizard of odds.