Why are adults telling other adults to 'use their words' on this thread? Why are women infantilising other women? Bizarre behaviour.
In answer to the OP, I completely understand and it is so difficult to extricate yourself when someone is in full flow - it IS rude to stop someone talking and not as easy to do as people claim. NT volunteers do need to be better trained but at the same time there's something lovely about people wanting to share their passion, isn't there? You don't want to curb volunteers' enthusiasm, but striking a balance in their interactions with visitors is vital so as not to irritate and actually detract from someone's experience. It's a fine art and I'm not sure it's something you can actually teach, as it relies on the ability to read people.
I'm a frequent NT visitor and when I'm bored of listening to someone telling me about a piece of tapestry I'm not interested in or whatever, as soon as there's a pause in their flow of information, I say 'You're so knowledgeable, I could listen to you all day, but I'm afraid I've only got half an hour and want to make sure I see as much as I can, so I'll have to move on now. Thanks so much' - and walk away briskly. Works every time and no one's feelings are hurt in the process.
The problem with many volunteers is the assumption that you don't know anything about the place you're visiting or the period of history it represents, which I find bizarre when you'd surely operate from the perspective of expecting the people who visit to have an interest in the place/period of history to start off with, otherwise they wouldn't be there. The best tour I've ever had in a heritage attraction was at Cardiff Castle, where the lovely tour guide asked us all what we already knew and what we wanted to know more about before we started. I said to him how rare this was and he told me he'd been humiliated when he first started by lecturing a man about a particular battle or something, and then the man, much embarrassed, had to reveal to him that he was actually the world's leading expert on that battle and was a Professor at Harvard, on holiday. From then on, the volunteer said he always checked first before he started sharing information. I think all NT volunteers need to be told that story as a cautionary tale - ask first!