In my search for a solution I've found that sometimes when there's a hashtag for a page anchor, pressing back once reloads the page without the hashtag, the second press of back takes you to the previous page as expected. I don't have the problem so I can't see what's happening to the url, hence the question.
Ummmmm.... this might just be what's supposed to happen. If you go to some page or other, and in that page there's a link to an anchor within the same page, by clicking on that link you (a) change the URI in the location bar to the URI of the anchor (which as far as the browser is concerned is a different URI, not the URI of the page), and (b) insert the 'plain' page URI (the one without the '#anchor' bit) into the history list for subsequent retrieval by the 'back' button should you choose to use it.
If some page or other has a link to a URI which is a direct link to an anchor within a page, then I'd expect the 'back' button to take me back to the page with the link, not to the (so to speak) 'top' of the page containing the anchor, which I would call incorrect behaviour. Is the incorrect behaviour what you're seeing? If so, I'd suspect some sort of page coding issue, perhaps something with frames and/or JavaScript. I'd be a lot happier if JavaScript and frames had never been invented, but then publishers' options for abusing HTML would be much more limited.
Incidentally I can't remember seeing this behaviour on BCUK, but I've seen it occasionally on other sites. Usually I just don't go back there, like when people put slideshows on their home page or ask me to install something to view their site 'properly'.