I, too, am not planning to take this lying down!
I've had a look at a few employment tribunal cases which have involved similar circumstances, and more often than not the judgement is that there is no legal entitlement to backpay if you have left employment. So, unfortunately, it appears there is nothing black and white set down in law to cover our predicament.
However, I also found one case where the claimant won their case because the company they worked for had always paid backpay to leavers, and so this was taken as the default position, but then on a whim the company decided one year that they didn't want to pay it. To me, this sounds remarkably like the stance Network Rail is taking, but before I can be sure, I would need to know whether Network Rail has paid leavers in previous years.
For anyone would would like to have a read through the case in question, it's here:
Employment tribunal case
I would draw you attention to the following:
Paragraph 57 states: "
Payment properly payable for work done prior to the termination of a contract of employment is payable as wages under that contract regardless of when they become payable."
Paragraph 61 states: "...
the amount payable is, in the absence of any other contractual terms, the amount as agreed for a particular year, regardless of when that agreement was reached."
Paragraph 72: "
It is not disputed that the payments were increased for employees who had worked from April 2022 and were continuing to work for the respondent. That can only be because the respondent recognised that it was money owed in respect of work done by those employees from April 2022 to November 2022."
Paragraph 73: "...
those retrospective payments referable to the period from April 2022 to November 2022 can only be retrospective payment owed, under the contract, for work done during that period. The fact that it only became payable after the end of the claimants’ employment because of the delay in negotiations does not mean that it stopped being wages."
Paragraph 74: ".
..the increased rates of pay were referable to a period when the claimants were working under their contracts of employment and the rate of pay to which they were entitled for that period has been increased accordingly."
Paragraph 81: "
The date on which the obligation to make payment arises is on application by the claimants in accordance with this implied term."