Exhpart, i don't know if it'll apply to Sheffield, but i looked at the Wembley "new member" presale today, and if it's any help:
they offered that deal where you buy tickets and a new membership at the same time.
(the drawback to that of course is that it's only good for one transaction, even if it's for less than four tickets -
the pre-existing codes are supposed to let you buy [for example] two + two or one + three.)
the best tickets i saw using that option were B4 row 11, for 150 GBP; most of what i was offered was in the C sections.
"What do you want - what?!"
- Keith