Several years back I read The Hobbit and Lord of the Rings.
I recently discovered (when going through some old boxes I hadn't unpacked after moving house) that I've got a few additional books which I've never read, mainly The Silmarillion and The Book of Lost Tales Pt. 1 + 2.
Is there any order to these? Which should I read first? Or doesn't it matter?
I asked a similar, more general question on Literature.SE about the reading order of all of Tolkien's works. The consensus was that The Silmarillion should come before the less mature HoME. The Silmarillion is a few well-developed stories about the First Age, while The Book of Lost Tales is a collection of story fragments and edited notes covering some of the same content. By reading The Silmarillion first you will have a better understanding of the history as Tolkien intended us to read it (though even The Silmarillion was not complete at the time of his death), then you can go back to Lost Tales and see how the stories developed from earlier notes.
I would recommend reading The Silmarillion first. That's the order of publication, and The Book of Lost Tales contains only fragments of stories that will act as spoilers for the more coherent story contained in The Silmarillion.
I'd agree that you should read The Silmarillion first, but would just add that you should probably read Unfinished Tales next, before the two BoLT volumes. UT includes additional versions of several of the stories from the Silmarillion, as well as a whole lot of extra information about Tolkien's world (including who the Wizards really were).
Besides the order everyone suggests, if you've never read the lorebooks, I would suggest you have a browser handy with the Encyclopedia of Arda on it...because you likely WILL get lost in the names and details.