The Sunday Times reports that the IMF could be an alternative source of funding in the event of a No vote.

One point to keep in mind in this debate is that the IMF charges a penalty premium of 200/300 basis points on large loans, whereas the premium has been dropped from EU loans, as decided at the July 2011 summit.

The relevant IMF funding schemes  (Extended Fund Facility, Precautionary and Liquidity Line) are described here and here.