I currently (not my choice) have both models, RX300 and RX400h. Is your RX a series 1 or 2? Your mpg figure for your RX300 does not surprise me. You will get better mpg from the 400h. For comparison, I am getting roughly 17 mpg for local town work of less than 2 miles. The 400h does about 24 mpg.
My advice (from experience) would be to keep the RX300 and use the £4,000 to run it. If your RX300 is sound then .............. "Better the devil you know"
If someone was migrating from another make to a Lexus RX, then I would recommend the 400h over the RX300. Work out your mileage and see how much EXTRA the 300 will cost you. The psychological difficulty is seeing/working out the mpg as you fill up. In terms of money, you would be spending approximately £1,150 more for fuel every 12,000 miles. That would take you about 4 years to break even - 7 years if you did 6,000 miles a year.
The 400h's are reliable but issues aside from the battery can be costly. What would appear economic sense is not necessarily what we would actually LIKE to do!
Please let us know what you decide to do. Your decision is the right one for you.