Most companies charge a fixed price per socket change of around £25 per socket
Always advisable to have an electrician do this in case there are additional faults, if you started had issues then got an electrician, the price would increase significantly so it just isnt worth the risk also there is the potential of voiding your home insurance
If you mean simply to take off the old socket and replace with a new one then expect to pay from £25-£45 per hour. You may get someone to do it at a fixed price. Yes you need an electrician as they will be able to advise on and rectify any faults if they find them.
Sometimes when you take an old socket off you find all sorts of issues.
If it was me I would charge £45 for first hour and £25ph thereafter.