I would say, discuss the potential salary at a second interview or when asked, but I wouldn't ask about it until at least a live, in person interview. If following a phone interview, a potential employer asked me to come in for a live interview and there had been no mention of salary, I 'd probably say that while I don't want to suggest we negotiate salary at this time, my current salary is in the range of X, and it they think the position would be able to offer more than that I am very interested in moving forward with the live interview. It seems like a total waste of time for everyone if they are thinking of one number and the candidate is thinking about something totally different. It's harder if you are not working to have that conversation.
In the case of a narrow range, 7 vs. 8 pounds, I'd interview and then try to negotiate up, since for a part time job, the difference between the two is not going to be that great in the end.