I made Mr M test drive it for me
You could ask for them to pay the road tax, or throw in a tank of fuel, or maybe some mats or something.
With it being so new I wouldn't expect anything to need doing, but if it did I'd get them to do it. For example my car was advertised at 3750, we grudgingly agreed to pay that, as it was the best we'd seen for miles around (and we looked hard) but a couple of the parking sensors weren't working, and MrM knew that the cambelt and waterpump would be due soon, so we got them to do that and fix the parking sensors (so that saved us quite a bit as we'd have had to get them done anyway).
You should be able to get them down a couple of hundred at least, I'd have thought.