Still no response from potential referees... Suggestions? About a week ago, I emailed three of my MA tutors to ask them if they can write me a letter of recommendation for my PhD application. No one has answered yet, though. I know that tutors are busy people, but really, it was just a "yes or no" answer. In addition, I explicitly stated on my email, that if they didn't feel comfortable writing a LOR for me, they shouldn't feel obliged, etc. 

My question is: 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? 

Thanks for your help, it's much appreciated!

P.S. I know what I'm asking sounds a bit funny, but I'm really at a loss here, cause I definitely don't want to be annoying, but without references from my MA tutors, I probably can't apply for a PhD, at least to some UK universities.
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.