A - shaft that operates the switch and lock bolt; B - keyway for switch location; C - rubber cover for switch; D - Key number; E - Shear-bolts; F - Lock bolt; G - location boss around bolt.
Drill up into the old shear bolt shanks (new lock pictured for clarity). Note that this can only be done with the earlier front-entry locks mounted lower down the column, not the later side entry accessed through the cowl as the bolts are in line with the lock instead of being at right-angles to it.
Parting the multi-way connector with outside circlip pliers, to get the indicator switch harness out of the way
Remove the switch
Old lock removed, I'd managed to drill right up the middle of one bolt and only slightly off to one side of the other.
When fitting the upper part of the lock clamp, the hole A seems to be positioned on the key-side of the fixing holes. Whether this is important or not I don't know.
When fitting the switch back into the new lock, having the clamp loose so the lock hangs down (gaps at A) makes it easier to access the switch fixing screw (B). There is a circular hole in the column outer tube (C) and a corresponding boss around the bolt in the lock body, to ensure correct positioning.