Sticking in "graduate AND C++", it looks like it's circa £23k and up, depending on location and industrial sector.
The financial sector pays the best, but it's not everyone's cup of tea. And the big banks are shedding a lot of jobs at the moment, so it's prob. harder work than usual to get a job in that sector.
The highest annual salary I could see was for a quantitative developer, which was up to £300k. I gather the top rate for this kind of role is even higher, but you won't see ads for the top positions: they're all head hunted!