If you are taking on more responsibility you should get paid more. Manager's pay is often based on their level of responsibility and skill to handle things when there is a problem. If you will be assuming her role in total then you deserve a larger raise. If you will be just be required to perform higher level tasks then you should get a medium increase. However if you continue working at the same level as now, you should get something for giving up a half hour of paid lunch and working more than than 40 hours a week. It depends if you get paid for your lunch period. You should develop your negotiating strategy based on how much you need the job and how easy it would be to replace you. If you cannot get all of what you want at first, try to get a written commitment, e-mail should be fine, that you will get another pay review after 90 days in your new role. You will then have the leverage of them depending on you and proven performance. Good luck

I think you have to consider that you have less experience and are only 22. If I were you I'd accept what they offered until I had actually done the job(s) for a year and then once I knew I could do what is proposed talk about a rise.

You say it is a small company and not doing very well at this time so why rock the boat? It is not anything to do with age discrimination it comes down to what you are worth to the business.

You cannot use the argument that you will be taking on a huge workload if you have volunteered to take on extra hours and do two jobs.

I think that £7.70 ph sounds reasonable as a starting point, and regular pay reviews should be agreed in order to increase that rate if your work warrants it. However, the company is not doing well and you do not have the experience that your previous manager had, so your boss is going to use both those things to keep your pay as low as possible. I think that you have made a mistake in threatening to leave before a pay offer has been confirmed - you would have been better to refuse the promotion due to insufficient pay and then look around for a new job. A boss never likes to be threatened, especially when he has not yet made an offer.

A pay rise from £6.04 to £7.70 is an increase of 27% - a bit optimistic for someone with very little experience even when a company is doing well, let alone when it is struggling. If you settle for £7.50 you are still getting an increase of 24%, which is pretty chunky by any standard.

I appreciate you feel undervalued but whether or not you can pay half the bills if you move in with your boyfriend isn't your employer's problem, however infuriating for you. Settle graciously for the best deal you can get, don't make empty threats (if you say you'll leave if you don't get something, it's a bit of a hostage to fortune) - but start looking for a job which will pay you what you believe you are worth.