I think your struggle with them Autotrader prices if Im honest, esp if your after a quick Sale and this time of year (after Xmas).
The 1.2 are the more popular of the range, as in, more of them, what Colour ?, Black, Grey and Silver are good selling colours for Corsa C's, the Design are pretty basic, SXi and SRi demand more money if looked after and in good nick. Lots of Corsa C's getting scrapped at the moment, due to poor maintenance, typically lack of Coolant and Oil Services, not saying yours is Varche, but as a general guide high number of the Models getting Scrapped or Broken normally means the Ones on the Road suffer in Retail.
Positive points they are low Tax and Insurance and cheap to fix, so always a good choice for New and Young Drivers, which in that Category a 13 year old Corsa C will probably Sell better on its looks, Colour, condition and MOT life over the the normal Millage and Service History.
Good luck with it Varche, I'll put the word around when you come up with a guide price.