Ouch. That's gonna hurt a LOT of people. Not just Northern Rock. But expect them to be a takeover target anytime, well, now, really.....
Normally banks have in place what are known as "Commitment Facilities" with other institutions to avoid this kind of thing happening.
A CF basically works that, the borrower (Northern Rock in this case) can borrow up to a fixed amount (set int he original contract) at a fixed rate of interest (set at the time of the borrowing) on demand. Under the terms of the Facility, the Lender is obliged to give the loan on demand from the Borrower. Normally this would be done with about 1 hours notice.
In return the Borrower pays a % fee to the Lender based on the un-drawn amount.
So lets say Bank A has a £500m facility with Bank B. On demand, Bank A can ask Bank B to lend them any amount up to £500m on demand. Bank B are contractually obliged to do it.
If Bank A doesn't borrow anything, it pays a fixed fee (say 0.125% for example) on the undrawn 500m, ie 500m x 0.125%.
Lets say Bank A borrows £200m, it then pays interest - normal market interest rates - on the £200m borrowed and the standard 0.125% on the 300m not borrowed.
So the fact Northern Rock have got as far as the BoE means they've *really* exhausted their funding options both on the market and in terms of their commitment agreements.
It might well only be temporary, but it does show NR haven't arranged their funding and hedging well enough to protect themselves in even the short term (<12months), never mind the mid (12m - 5 years) and long (5 years+) term.
Ouch.
Real, big, ouch.