Is a claimant entitled to disclosure of previous claims or complaints against a professional on the basis evidence of past transactions show a propensity to negligence?
The House of Lords set out the test for admissability of “similar fact” evidence in civil proceedings in O’Brien v Chief Constable of South Wales Police  2 WLR 1038. In that case, the claimant alleged the police had used oppressive, dishonest and unprofessional methods to extract a confession from him. The claimant sought to support these contentions by adducing evidence to show that the officers concerned had used similar methods in earlier cases.
The House of Lords set out a two-fold test:-
a) Whether the evidence in question was relevant, i.e. potentially probative of an issue in the action; and, if so,
b) Whether the evidence should be admitted in accordance with the overriding objective, including weighing the significance of the evidence against its probative value; the risk of prejudice; and proportionality.
Documents that go solely to credit are outside the scope of standard disclosure (Favor Easy Management Ltd v Wu  1 WLR 1603 applying Thorpe v Chief Constable of Manchester  1 WLR 665).
Is such evidence relevant?
It is quite common for professionals to be the subject of complaints or indeed claims, relating to a range of matters from delay to inadequate professional service. Does the fact that a complaint or claim has been made against a professional really make it more likely that he was negligent on a specific occasion? It could even be argued that a previous claim would make the professional more careful in future.
The Court of Appeal (admittedly obiter) actually considered this point in Thorpe. Dillon LJ (with whose judgment Mustill LJ agreed) stated in terms that such evidence would not be relevant:-
But in an action for damages for professional negligence against a solicitor evidence of other claims for negligence made or established against the defendant by other clients in respect of other matters would be irrelevant and inadmissible and discovery in respect of such matters would be oppressive; a plaintiff charging a solicitor with negligence in one matter could not investigate other areas of his practice in an endeavour to establish that he had a propensity to be careless. (at 669G)
Given this, it seems to the author that, to be admissible, such evidence would have to go far beyond evidence allegedly showing a “propensity to be careless” and really be evidence of very similar conduct to that complained of in the action concerned, closer to the sort of evidence that has been allowed in police cases (see above).
Further, in a given case evidence of previous claims or complaints was held to be relevant, there may well be grounds for excluding such evidence under the second limb of the test. Widening the scope of the trial to include other unrelated transactions is likely to add significant expense and time to the proceedings and there would certainly be arguments that incurring costs dealing with these transactions would be disproportionate.