I would still go for the underlay, the problem here is that the more you win the bigger the wager requirement it seems. So if you back £25 @ 10.5 you will win £237.50 profit that you will have to turnover 5 times.
That's the way I read it from their T&C's anyway.
I'm not sure how I would play this one really but I know I would underlay. High odds do win!