It is a widely-established constitutional principle that no Parliament can bind its successors. So if the redundancy package was set out in primary legislation by Parliament as mentioned further up the thread, then it was implicitly subject to amendment or repeal by any future Parliament.

Under that circumstances, I'd say the naughty government was the one that agreed to the original terms, unless of course their negotiators explained this point to the unions at the time (mind, you'd think Civil Servants ought to be aware of the issue)