I think I'd walk if all I could get off a £30k car was £500, unless it was abolutely spotless (at which point, I'd probalby not look for any redcution). Trouble with lot of cars is that my idea of "spotless", "very clean" or "very well cared for" is different to pretty much every seller I've come across.
With a GS, you only need find one or two small things that will need addressing in the next few months and £500 is easy to spend.
With regards to negotiations, I know of at least one dealer who drops a grand a month off when the cars dont sell, up to about 2-3 months in....that there makes me think theres room for negotiations!