To ask for a raise because I know what other people are on?

[DerelictWreck]

My company have offered me a 'promotion' (shed loads more work and no option to refuse it as I didn't apply in the first place) and have offered me a 10% bump along with it.

At first this seemed fair enough (10% would take me from £30K to £33k) but I have since found out that two of the people who have just left from my new 'hierarchy tier' but were on £36K and over £40, despite the company not viewing them as good at their jobs (why they left), and one of the women still on my new 'tier' is on £41K.

They do all have more experience than me, but are widely viewed by the senior team as a bit incompetent, and it is some of their work that I am taking over. Based on this, I feel that they are low-balling me as they know £3K extra is useful to me, but I'm not sure how I can negotiate it as I can't reveal that I know their salaries!

Any thoughts on what (if anything) I should do?

I would negotiate the terms with them. Pretty standard process. You don't need to say you know other salaries, focus on what you feel you and the role are worth, noting the increase in responsibilities and your skills to meet the task. Good luck!

Rather than say you know their salary can you research what other companies are offering for that role, find out via job adverts?

You don't need to mention other people's salaries, when you are negotiating for a raise you have to prove why you are worthy of the extra money you are asking for, which could be any % of your current salary. The fact you are taking on more work is a good starting point. Good luck!