No, its definitely not the Duchess of Gloucester.
I am sure Warren or Marsel will know who this is
Sorry Zonk, we'll have to rely on Warren to indentify this lady.
Duchess of Gloucester was my best guess, although I admit the features aren't quite right.
I can speculate with the known facts a bit though: since she is seated
after Gordon and Sarah Brown but
before David and Sam Cameron, and in very close proximity to the Royal Family, I'd say she's either a high-profile member of the Labour party or wife of one (or a noblewoman / member of the extended BRF).
Of high profile female Labour members, I can only think of
Margaret Beckett,
Harriet Harman and
Tessa Jowell. The woman is definitely not Beckett but she could be Harman (same hair colour and profile) or Jowell (similar profile, slightly different hair colour).
Of wives of high-profile Labour members, that could be either
Louise Shackelton - David Miliband's wife (unlikely, since Miliband was seated next to Prince Charles so presumably, his wife would be somewhere there as well) or
Alice Perkins - Jack Straw's wife (much more likely, because the man on her right resembles Jack Straw quite a lot). I don't see much resemblance to either of them though; besides, Alice Perkins is Companion of the Order of the Bath and would most probably wear the Companion's badge for the event (which the lady in blue clearly doesn't).
The lady is definitely not spouse to any of the non-Royal Dukes though, that I can guarantee.