Underpaid examiner's wages not raised sufficiently to cover responsibilities...

I have a question about equal pay. In July 2006 I was recruited by an exam board as a Junior Test Developer. I was offered £26,000 after saying I would accept £25,000 in an interview. I have 5 years teaching experience and 5 years as a grade A marker for the papers I was recruited to design. At the time I (wrongly) presumed that those above me would have superior expertise/subject knowledge. Another colleague who is also female and 16 years older than me was recruited as a Senior Test Developer. She had 1 year teaching experience and little or no experience on the exams we were designing. She was an internal appointment. I have since discovered that when I started she was on approx £36,ooo! After 3 months in my post I was promotoed to Senior test developer like her as it was clear I had the skills and experience. I was offered £31,000. I accepted as I did not know what she was earning at the time and it seemed like a big upgrade. However, over the past 12 months it has become clear the she doesn't have an aptitude for the work and has resigned. I am very annoyed as I know she has earned thousands more than me in the past 18 mths - even though I have worked harder and achieved more in the same job. To add insult to injury they have advertised the job for CIRCA £35,000 dependent on skills and experience!! I'm beginning to feel very ripped off by my company and requested a pay rise. They offered me a lousy £1900 and I turned it down immediately. They had my job externally benchmarked and have said the outcome is 'positive', although for some reason they are keeping me dangling on the actual figure. Do I have a legal right to request the rise be backdated to when I first requested it at the end of October 07? And do I have a chance (though I doubt it) of recouping some of the earnings I feel I was conned out of over the past 12 mths? I'd really appreciate some legal advice on this. For the record I wish to stay with my employer for now, but am looking around for similar work with another exam board which will pay me fairly and not take advantage of my naivety!