1. You get locked into a two year contract, stating you can't leave. If you do leave within the two years, they'll fine you £12,000. How enforceable this is, I am not sure, but I didn't take the risk
2. Salary doesn't really grow that much even after two years. I started off with £26,000 and now I am £30,000 after two years :(
3. This isn't really a consultancy for experienced software engineers. Its for grads. Once you do your two years, probably best to find another better paying job.