Thanks for your reply. A couple of points.
I had a look at a torpedo level and cannot see the advantage to it over a regular level [which I own] placed between the drain and the wall. Will the 45 degree angle level that I see on the torpedo diagram rotate in some fashion? Or is it the shortness of the level that is useful, i.e., that it will fit within the fairly short distances to be found in a shower pan.
Can I just use the thinset mortar I used to set the tiles as a 'mortar skim coat', to use your expression or should I buy some kind of special leveling mortar?
Any tips on the actual chipping would be appreciated, as well.