Any safety event (stop short, excess speed, Cat A SPAD) are recorded in someones safety file forever, and in most TOC cases - on an electronic file system called CMS.
The B1 form (old ATOC form as ATOC no longer exists) species you must detail every safety event regardless of time passed. Below is a copy and paste from the current form:
5.1.1 Have you previously been involved in any SPAD incidents (including incidents where you were subsequently exonerated)?
5.2.1 Have you previously been involved in any of the following (including incidents where you were subsequently exonerated): excess speed (including TPWS interventions), TPWS activations, station overruns, failure to call, stop short and release of doors, wrong side release, collisions, derailments or other safety incidents?
5.3.1 Have you ever been removed from train driving, or had restrictions placed on you, because of a safety of the line incident, your health or other cause?
So whilst you have to decalare everything, it is upto the potential employer to consider each incident on merit and decide whether it would impact any application.