Thanks for the feedback so far, folks, in particular the informed technical opinions from John in Kansas!
So the situation seems to be that it shouldn't cause any particular problems, but as long as the computer already has SP2 installed and the regular patches for all the XP security holes, there isn't much in SP3 you won't already have covered.
I daresay I will get around to installing SP3, but if there is nothing REALLY urgent about doing so, I'll not bother this week.
Quack!
GtD.