Thought I'd bring a bit of closure to this topic: -
After getting passed about between independent mechanics and the dealer where I bought the car, I ended up going to the main Lexus dealer in Belfast who diagnosed it as a faulty MAP sensor.
Yes, this car does indeed have both the MAP and MAF sensors, the one @200h identified is correct. Lexus wanted £230 for the replacement sensor. I got a used one on eBay for £25, installed in 2 minutes and cleared the codes using a simple obd scanner.
Job jobbed.