Thanks for the comments Roger, I did have a suspicion that the barrel wasn't supposed to be unscrewed like that in use. I'm still not exactly clear on why a 12mm EP with a 2 x barlow for example would be "easier" to use than a 6mm EP - will I have an easier time focusing and viewing with this configuration? Won't I have halved my field of view?
There certainly isn't a shortage of options with eyepieces to choose from.. I saw an interesting video today on YT
http://www.youtube.com/watch?v=S5oIBZ1KD2g that appeals to me - as his second example is the exact pair of eyepieces I have. His suggestion is to consider either a 9-13mm or 15-17mm plossl eyepiece as a first supplement to the basic pair of modified achromat EPs that come with the scope. From what I've read here and elsewhere, that sounds like good sensible advice. I just have to decide which one (probably around 13mm) and settle on a brand that suits my budget. I'm also keen to get a moon filter pretty soon, I guess a variable polarizing filter would be a good choice for long term usability regardless of whatever I might get in future..
Just saw raymo's post as I was about to click submit - I was thinking about 2 inch eyepieces too - I'll have to look at those as well..