Thanks in advance for any advice

I have been on project secondments for 4 years & 7 months and each project secondment stipulates you move over on same terms & conditions.

However, is it right that I can still be paid a salary for a job I haven't done in nearly 5 years? Not to mention the new skills I have picked up since working on projects.

Raising this with line management is not helping as they just state that you move on to Projects on same terms. But is there anything in employment law or best practice guidelines that I can refer to after having been on projects for so long?

My job description is also still for a role I have not done in nearly 5 years.

You don't say where in the world you are, but employment law doesn't cover raises anywhere I know of. If written company policy dictates salary ranges that match job description, that's strictly a company matter. The whole world is still in a recession to some extent, so it isn't easy to get raises. If you think you can figure out a way to demand it based on your skill level and good work, do so. Sometimes this means self praise that most of us aren't accustomed to. Sell yourself! Say you deserve it and say why. If you want to take the risk of saying that another company is interested in you, do so, but be careful. That's only if you really feel indispensable.

Emplyment laws do not cover raises. Seems like you expect to be paid more because you are getting better at it, sounds reasonable right? Many employers appear to terminate employees who do not continually improve. Sad to say but that's their thinking