Still no response from potential referees... Suggestions?
Should I interpret their silence as a discreet 'No' or am I to assume
  that they've probably forgotten or the email slipped through the
  cracks, and send them a second email to remind them?


I think you should send them a polite follow up email. Even better, talk to them about it face to face or on the phone so you can get an immediate answer (and one that will be more likely to be positive). I had the same experience when I was looking for a reference. I didn't get a response but then when I followed up on it, it was all good. More generally, I find that busy people, such as academics, frequently let things fall between the cracks and that one shouldn't be too quick to assume the worse.