From AIP AERODROME AD 1.1 AERODROME/HELIPORT AVAILABILITY:
4.6 Altimeter Error
4.6.1 When calculating Decision Height (DH), account must be taken of the errors of indicated height which occur when the aircraft is in the approach configuration. Details of the Pressure Error Correction (PEC) should be available from the aircraft Flight Manual or handbook. In the absence of this information a PEC of +50 ft has been found to be suitable for a wide range of light aircraft and should be used. This addition of 50 ft need only be applied to DH. The required RVR should be calculated prior to applying the
PEC.
4.6.2 The use of a radio altimeter is only applicable to approved Category 2 and Category 3 operations. For an aircraft flying a Category 1 or non-precision IAP, DH/Minimum Descent Height (MDH) is indicated on the pressure altimeter. At DH/MDH any readings from a radio altimeter may be unreliable because of the large area of terrain providing return signals to the instrument.
since it must be applied (or 50ft in lieu) to an ILS, which I believed to be the topic under discussion.
Again, I have not disagreed with you.
Presumably, you are now going to introduce temperature corrections into the debate?