Terms of seven days are utterly reasonable, and the six months has nothing to do with anything.It's not worth putting into small claims, as by the time that goes through the system, she'll have paid; or if it gets there before she pays, she won't pay anything until the last possible minute, I would assume.
Simply tell the woman that you'll be adding an administrative charge for paying later than 7 days. Anywhere between £10 and £25 would be reasonable. You then have something to pursue (optionally) at court if she pays late. You might choose never to do so, but you have up to six years to make your mind up, and you could send her a letter to ensure that she spends the next six years wondering...