No that's incorrect if it was specified ST would be granted without time limits ... but it all depends upon the exact wording. Significant subsequent changes, such as time limits that were not existing when the original contract was struck could be seen as unfair and contrary to the
Unfair Contract Terms Act 1977
For instance, would the severance offer in regard to ST, have been accepted if a time limit had been included?
BUT ... it all depends upon the agreed terms at the time and how carefully it was worded.