What can I do about finding out the arrangements to be used when I cover the work of a more senior person?

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 organizations, should be available, in an accessible place, for you to read.

You may, if you wish, refuse to accept temporary promotion, although you should be careful to ensure that your contract of employment does not require you to undertake these extra duties.