For what it's worth, I would suggest that many people actively looking for a diesel GT will not be put off by higher miles. Yes, low miles will tend to mean less bits worn out, but you could take the view that higher miles means the bits have probably been replaced by now....
When I decided on a GT, I started off looking at the bottom of the market - 100,000+ miles, no leather etc - these were starting at under £3k
In the end, I got lucky and picked up a 2008 Blackline with all the usual goodies for a little over £4k, so the market is definitely in favour of buyers, not sellers. Poor dealer didn't know what hit him, but at least he shifted a car with a very limited set of potential buyers
At 4.8k for a 2006 car, I wouldn't have given it a second look when I was browsing. I accept that it's been well looked after, but most buyers are after a low price and will accept the occasional niggle that comes with a cheaper car.
Also, FWIW, I wouldn't entertain a GT without leather