In your situation, being already well known to the school, I would phone the person arranging cover and ask whether they only use subject specific supplies. Let him or her know that you are willing to cover other subjects. I only go to one school and cover whatever they want me for, since they often need a range of subjects covered in one day. Sometimes a subject that is not my specialism request me as I am quite familiar with the pupils, and sometimes the school gets in a specialist, eg for PE, or if they need cover for a few days. Just let them know you are willing, maybe for less than a whole day, which would be cheaper than having to pay an agency for a whole day.