If this is going to be a constant request set a charge either per item or a monthly fee. If as you say the changes are small, try £5 - £10 per item changed or a monthly fee of say £25. You probably wont get any business but at least it will encourage the owner of the site to follow your instructions and have a go herself. If you are going to completely redesign a site I would suggest you treat it as a new design and charge at least £75 - £100. Explain to any site owners that your time is of value and you need to be paid for it.