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.
However, 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.
Sometimes they will be covered by a collective agreement negotiated by your union.
You may in some circumstances refuse to accept temporary promotion. Consider whether there are safety grounds for refusing – for example, you may need to ask for extra training to be able to carry out the more senior role.
Check your contract carefully before refusing to undertake extra duties. It may contain 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."
It is sensible to take advice from a union rep or Citizens Advice if you are concerned.