Think you may just be a victim of dieselgate and a softening of the UK's love of diesels. I traded my GTD last January and broke even on my PCP, after about 2 years (GFV £13,225, settlement £15,319), but things were on the change, even then.
GFV's are just an educated guess, with an overriding slant on encouraging business. The perceived wisdom of early 2014 was that diesels were the eco way to go, with increasingly better emission controls and, therefore, rock solid residuals. This has all changed massively with the new recognition (on this side of the Atlantic, anyway) of the diesel NOx emission problem.
How far out are you, anyway? Ah, just read your later post and given my figures, above, I can see your point. Seeing that your deal was done around the same time as mine (March 2014), it is difficult to imagine why the 2 GFVs should be over £2K different! Hope you manage to negotiate a deal.