Thanks for the advice (so many folk looking at the forum at this time on new year's day??!!).
I guessed that at that price Rotagrip are unlikely to make these gauges in-house. But as you say, their reputation seems good so one would hope the quality's there. That said, a bore gauge is a comparative tool, so as long as I use the same one all the time and set it up properly I hope I'd be OK.