The judges written verdict is recorded in Pensions Law Reports for 1998.  It costs hundreds of pounds a year to subscribe to these reports. The Times of 14/1/98 summarised the case, and here is how the summary started:

Pensions agreement enforceable

South West Trains Ltd v Wightman and Others

Before Mr Justice Neuberger

[Judgement December 18]

An agreement between a trade union and its members' employer in which hitherto irregular and non-pensionable payments were restructured as regular payments, on the express terms (a) that only a part of them should count as pensionable in the future and (b) the balance should not be pensionable at all, was lawful and capable of enforcement by the employer.

etc.  [We don't have the full text electronically but can offer a paper copy to those with an appropriate interest.]

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There is also an account written by the legal firm Freshfields but since Freshfields were solicitors acting in the actual case you might be cautious about whether that is a balanced summary.