Ah, don't worry Daisy. Babies are very soft and don't generally get hurt by rolling off something. Its because they don't tense up. I believe that because I have to BTW!
Oh Jammy, I don't know what to suggest other than can you have a duvet day? How about preparing a nice day in bed tomorrow? A good book, the newspapers, DH on hand to bring lo to you for feeding, or having her in bed with you? It really did the trick for me last week. DH could take her for a stroll and you could have a lovely long hot bath. Perhaps he could visit a nice deli and bring you some delicious bits back for lunch.
I may be projecting my ideal day on you, but you get the idea!