Thanks for entering the discussion, because although I felt sure a situation was being described that had never actually happened, only an insider would know for sure.
Am I right in thinking that a 100% change of the balises took place, clearly those fitted now are physically different (shorter and fatter) compared to those originally fitted?
When comparing the SWT and SN versions of the 'Tracklink' system, I feel the question of GPS vs Balise is a bit of a red herring, because on SN's version they also use a balise to override GPS wherever it is necessary to differentiate between different adjacent platform lengths, or whenever the GPS is 'shielded' - such as in a main terminus like Victoria. So in comparison, SWT's version is simply skipping the GPS location stage and going straight to the more specific track mounted balise.