We start with Jacobs v. Great Yarmouth and Waveney Health Authority  6 Med.L.R. 192 (CA), there Griffiths L.J. said (obiter) of the doctrine res ipsa loquitur that it,
… means no more than that, on the facts that the [claimant] is able to prove, although he may not be able to point to a particular negligent act or omission on the part of the defendants, the fair inference to draw is that there has been negligence of some sort on the part of the defendants; but that is an inference to be drawn upon the facts presented by the [claimant].
If there is further evidence presented by the defendant, those facts may be shown in an entirely different light and it may be that at the end of the day it is not possible to draw the inference of negligence.
In Ng Chun Pui v. Lee Chuen Tat  R.T.R. 298 (PC) Lord Griffiths (as he now was) returned to the topic with the following words,
So in an appropriate case the [claimant] establishes a prima facie case by relying upon the fact of the accident. If the defendant adduces no evidence there is nothing to rebut the inference of negligence and the [claimant] will have proved his case. But if the defendant does adduce evidence that evidence must be evaluated to see if it is still reasonable to draw the inference of negligence from the mere fact of the accident. Loosely speaking this may be referred to as a burden on the defendant to show he was not negligent, but that only means that faced with a prima facie case of negligence the defendant will be found negligent unless he produces evidence that is capable of rebutting the prima facie case.
See the other posts under the res ipsa tag especially Ratcliffe.