I think it depends on the people constituting the body of the society. I can imagine it being as you, Lucy, describe it at the upper tier universities - and Sheffield is very much upper tier. Through both my work and more recently my own student expeience I've been able to see and feel first-hand the difference in "attitude" and I can imagine why you would feel "intimidated".
I don't really know what to suggest. I've found the society members at Hallam to be absolutely fantastic. Very, very, very inclusive. I'm 35 years old AND ONLY PART-TIME- I'd left school before most of them were even born -but from day one thay have made me feel very welcome. I think sometimes though you really do have to "bite the bullet" and push yourself forward. I can quite appreciate why you would feel the way you feel, but you honestly might be surprised. Perhaps the best way forward might be to email the Master of Moots and enquire about getting involved - email being the easy, impersonal option - if you get cold-shouldered by email then your gut feeling is probably right; I suspect you might, however, be surprised.