It's not a dealer it's a private sale, he's only ever had one key apparently. He's a nice guy, director of an IT software company. (I checked him out on Linkedin), and he's had it a couple of years. Normally you get a feeling with a sale, he seemed completely genuine, he's also replaced all four Bridgestones, replaced the radiator as he noticed a coolant leak, and has had new discs and brakes fitted by Lexus.
I've just found the review on evo https://www.evo.co.uk/lexus/is/6815/lexus-is-f-road-test-review, I didn't realise it had an LSD.