Been browsing forums for a few days now and considering getting an early plate IS200 because the prices have come down a lot in the last few months and aren't much more than the Golfs I was looking at. The customer service, on the whole, seems to be pretty high with Lexus so I would prefer to buy a used from them but obviously not if it's going to be 20% extra. So how much can I realistically expect to get knocked off a 9k car?

from a lexus dealer you will struggle to get £500 of a 9K car but start at a grand and go from there .Good luck

Best time to try them is the last week of the month before their monthly financials close out. If they are a bit light on sales over the previous 3 weeks they are more likely to drop their pants on pricing.