A well regarded decision where Robert Walker L.J said at 2443,
Without in any way underestimating the need for proper analysis of the rule, I have no doubt that busy practitioners are acting prudently in making the general working assumption that the rule, if not “sacred” …, has a wide and compelling effect. That is particularly true where the “without prejudice” communications in question consist not of letters or other written documents but of wide-ranging unscripted discussions during a meeting which may have lasted several hours.
At a meeting of that sort the discussions between the parties’ representatives may contain a mixture of admissions and half-admissions against a party’s interest, more or less confident assertions of a party’s case, offers, counter-offers, and statements (which might be characterised as threats or as thinking aloud) about future plans and possibilities. As Simon Brown L.J. put it in the course of argument, a threat of infringement proceedings may be deeply embedded in negotiations for a compromise solution. Partial disclosure of the minutes of such a meeting may be, as Leggatt L.J. put it in Muller v. Linsley & Mortimer  P.N.L.R. 74 , 81, a concept as implausible as the curate’s egg (which was good in parts). As it happens, the minutes of the Frankfurt meeting are exhibited in redacted form in which the redacted parts of the document appear to amount to about 90 per cent. of its contents.
Nevertheless, there are numerous occasions on which, despite the existence of without prejudice negotiations, the without prejudice rule does not prevent the admission into evidence of what one or both of the parties said or wrote. The following are among the most important instances.
(1) As Hoffmann L.J. noted in Muller’s case, when the issue is whether without prejudice communications have resulted in a concluded compromise agreement, those communications are admissible. Tomlin v. Standard Telephones and Cables Ltd.  1 W.L.R. 1378 is an example.
(2) Evidence of the negotiations is also admissible to show that an agreement apparently concluded between the parties during the negotiations should be set aside on the ground of misrepresentation, fraud or undue influence. Underwood v. Cox (1912) 4 D.L.R. 66 , a decision from Ontario, is a striking illustration of this.
(3) Even if there is no concluded compromise, a clear statement which is made by one party to negotiations and on which the other party is intended to act and does in fact act may be admissible as giving rise to an estoppel….
(4) Apart from any concluded contract or estoppel, one party may be allowed to give evidence of what the other said or wrote in without prejudice negotiations if the exclusion of the evidence would act as a cloak for perjury, blackmail or other “unambiguous impropriety” (the expression used by Hoffmann L.J. in Forster v. Friedland (unreported), 10 November 1992; Court of Appeal (Civil Division) Transcript No. 1052 of 1992) …this court has, in Forster v. Friedland and Fazil-Alizadeh v. Nikbin (unreported), 25 February 1993; Court of Appeal (Civil Division) Transcript No. 205 of 1993 , warned that the exception should be applied only in the clearest cases of abuse of a privileged occasion.
(5) Evidence of negotiations may be given (for instance, on an application to strike out proceedings for want of prosecution) in order to explain delay or apparent acquiescence. Lindley L.J. in Walker v. Wilsher, 23 Q.B.D. 335 , 338 noted this exception but regarded it as limited to “the fact that such letters have been written and the dates at which they were written.” But, occasionally, fuller evidence is needed in order to give the court a fair picture of the rights and wrongs of the delay.
(6) In Muller’s case …one of the issues…was whether the claimant had acted reasonably to mitigate his loss in his conduct and conclusion of negotiations for the compromise of proceedings brought by him against a software company and its other shareholders. Hoffmann L.J. treated that issue as one unconnected with the truth or falsity of anything stated in the negotiations, and as therefore falling outside the principle of public policy protecting without prejudice communications. The other members of the court agreed but would also have based their decision on waiver.
(7) The exception (or apparent exception) for an offer expressly made “without prejudice except as to costs” was clearly recognised by this court in Cutts v. Head , and by the House of Lords in Rush & Tompkins Ltd. v. Greater London Council  A.C. 1280, as based on an express or implied agreement between the parties. It stands apart from the principle of public policy (a point emphasised by the importance which the new Civil Procedure Rules, Part 44.3(4) , attach to the conduct of the parties in deciding questions of costs). There seems to be no reason in principle why parties to without prejudice negotiations should not expressly or impliedly agree to vary the application of the public policy rule in other respects, either by extending or by limiting its reach. In Cutts v. Head  Ch. 290 , 316 Fox L.J. said:
“what meaning is given to the words ‘without prejudice’ is a matter of interpretation which is capable of variation according to usage in the profession. It seems to me that, no issue of public policy being involved, it would be wrong to say that the words were given a meaning in 1889 which is immutable ever after.”
Note – Muller’s case is to be read with care in the light of doubts later expressed in Ofulue.
This summary was quoted from extensively in Oceanbulk (see separate entry under this tag).