If you are asked to stand in for a senior colleague it would be usual for your employer to pay you the appropriate rate for that grade.
Much depends on the terms of your contract of employment and the organisation's rules for temporary substitution. For example, some employers will not pay for short-term substitution, say for a week or less, unless it is part of a continuing pattern.
The rules covering such arrangements, which will vary in different organisations, should be available, in an accessible place, for you to read.
You may, if you wish, refuse to accept temporary promotion. Consider whether there are safety grounds for refusing - for example if you have not been provided with the training you need to carry out the more senior role. You do need to check whether your contract of employment appears to require you to undertake these extra duties. Some contracts include a catch-all clause stating something along the lines of:
“the employee agrees to temporarily undertake such other reasonable duties as the employer may from time to time request.”
However, such a clause is not necessarily effective in all situations.
