working in the office/ doing office manager job whilst manager is not there - shall I be paid higher salary for these hours?

[goodbook200]

work in the office as an administrator and as I start work 7.00, my manager comes in at 10.00 till the evening. Between 7 and 10 I do all her duties, but still I am on my administrator's salary. shall I approach company and say that I feel I need to be paid manager's rate between 7 and 1 and then form 12 my rate?

Are you contracted to be in that early? Are you contracted to do those duties between those times? If so I think you'd have a good cause for asking for your overall wage to be increased, if not then you may have a bit of a fight on your hands depending on your employer.

in terms of list of duties - there is no list attacehd to the contract. All I have is the Job descitpion + Main tasks from the advert. Is it the valid document to look at?

Is this a new thing-was the manager previously in for all the time you were there?

Are you actually doing the manager's full duties, though? I'd be surprised if you truly were, including any live management issues.

If you gave something specific like key holder duties, as you're there early, then you might ask for something to reflect this.

have, not gave

what is live management please?

Live management is dealing with management issues as they arise, personelle issues being a good example.
When you signed your contract you should have been given a copy of that and the full job description plus any other company info to keep, maybe in a new starter pack, if not ask your HR department for a copy so you can check exactly what it is you are expected to do.
Be aware most job descriptions will state you are expected to take on other duties as they arise and is reasonable and they may think that this is what you are doing.

I think a lot will depend on what the manager is also contracted for, therefore are you actually covering her duties, as opposed to just cover the opening time of the office, for example.

If manager works 10-evening, then that sounds like a full-time role in itself - a whole days work, just later start and finish than you / others / normal.
I would presume the company's argument in response to your suggestion would be that the manager can undertake all her managerial tasks in her 10-7 time. Thus you're not covering for her - you just each carry out your full day's work in a different period.

OTOH, if you can establish an actual managerial task that you do BECAUSE there is no management prescence in those first three hours, then that's different.

Summary - doing a different working pattern doesn't automatically mean you're covering for someone else. It will very much come down to specifics - presumably, there ought to be administrator tasks you don't have time to complete because you are covering the manager? Just be careful you don't get caught out on that - if you have the time to cover someone else's role, then theoretically you're not "busy enough" with your own tasks.