I didn't quite know who to contact so on Thursday night I e-mailed Sirdar and explained how the colour was different from ball to ball, and hoped for the best. You can see from the photo below how the colour is slightly different, and unfortunately, enough to notice in my opinion.
I received an e-mail back Friday morning, saying that they were sorry I had found a problem and they would send me a prepaid label to send the yarn back for examination.
I didn't read the e-mail until later in the day, and as I had half knitted the hoodie I explained that I was rather reluctant to send anything off and would they accept some photos of the problem.
Due to the time I sent the e-mail I didn't receive a reply until Monday, and when they did they said that photos would do fine. I sent them over and within an hour they sent another replying saying that they would replace the yarn I had bought with any other yarn to the same value.
The moral if the story is that don't be afraid to complain as there are some very good companies out there with very efficient and competent customer service.