Well I went and had a look at it, had a test drive. And long story short, I bought it.
It was very nice apart from more wear than the pictured showed on the leather part of the steering wheel and a little ding in the left wing. They are going to fix the wing-ding (ahem) for me, which may be more complicated than they realise given it's aluminium ...but I made this part of the deal. Otherwise everything works as it should, the engine and gearbox seem fine ...I was very careful to check for rattles, knocks and other untoward sounds and a rough idle with or without the aircon on ...it has a full main dealer service history and drives very nicely.
It's so smooth and quiet compared to the 540i ...like driving a sofa

...well really it just feels like the other one I had only with a little less oomph from the smaller V6 engine. James May put it very well though when he said you just have to press the peddle a bit harder than with the V8

It has a very pleasing engine note, has a nice rasp to it when you give it some beans ...mostly though it's just very quiet and wafty.
The car has a few optional extras they didn't list in the advert because they didn't know

...it does have the Alpine premium sound system, which is nice, and an electric rear sun-blind, both of which I had in my last one. It has front parking sensors which the advert did mention and I didn't have before.
The only other differences are the fact my old one had nav, this one doesn't ...and this is a 3.0 V6 the old one was a 3.5V8 ...this one is what Jaguar call 'Winter Gold' ...SE spec ..with lots of chrome, old one was a Sport in metallic black without lots of chrome ...also the sport has a firmer suspension set-up and CATS active suspension, ...the SE's do not have CATS as far as I know. This one also has walnut wood rather than the darker stuff I had before and the seat upholstery is the same colour, a lovely cream. This one also has the genuine tan/beige coloured Jaguar over-mats ...the old one I had didn't.
I got it for £6,941 in the end ...not bad for a 53 plate XJ6 SE
I did take some pictures with my phone, although they aren't brilliant I'll post them up later when I've had something to eat. I've been on the go most of the day!
I should get the car sometime next week, depends how fast they can get it through the body-shop.
That makes no sense. It costs £200 to buy an amazing satnav. It won't be integrated, but so what? Window mounted satnavs are safer IMHO - at least your focus is always forward when you need to look at the screen, instead of down at the dash.
The properly integrated systems are a little more than just nav though, the touch screen computer system can control all sorts of things from weather the climate control buttons beep when pressed to to the voice control system visual interface etc ...also the dash looks better with it than without, more modern and with less buttons.