What do I have to do to get into the £35-40k bracket? I've had numerous interviews and a couple of offers, but none will move out of the £28-32k range.
But I don't think it's that unusual for a new graduate position in a non-software company. Also, compare with any friends from university in science etc and you'll hopefully feel better about it.
At least make sure you're learning new things. Testing? CI? Agile? Configuration management? Cloud deployments? That's the things I pursued, but there are many possibilities. Hopefully you have done freedom to explore on work time. Identify what's needed most and looks good, get it done. If it doesn't get you a promotion at your current job, you'll at least be able to talk about it with confidence at the next interview.