Spurred on by Tony M's assertion "often (but not always)" I did some more searching. I found this article
https://www.dailywritingtips.com/i-just-got-wise-to-“compris... which is actually on a different treacherous aspect of "comprise"... but two of the comments there are by patent lawyers (it's US usage but I presume - until otherwise enlightened - that UK/international English works the same way). Both these people are categorical: "An apparatus COMPRISING X, Y and Z means an apparatus having at least X, Y, and Z. Something having X, Y, Z, and K is within the scope." So there you go. I do question whether EN patentese necessarily aligns perfectly with everyday English in that regard, but from now on I'll feel less dodgy using "comprise" in patents.