Cheers chaps, much appreciated.
Dave - does the one I linked to look like yours - i.e. 3 point contact? If not, I'm guessing a 3-point is easier/more reliable than a 2-point (or 1 point + measuring plunger)?
e.g. the bore gauges I've got right now are the "T" shaped ones with spring loaded anvils, you stick it down the bore, lock the arms with the screw top, withdraw & measure; these rely on the spring loading to put them in the middle of the bore.