I have only managed to locate one fuel consumption figure. It's from "The Deltics, A Symposium" Second revised edition published by Ian Allen. The figure is from a test run in November 1955 and the results were published in BTC Bulletin No. 19. The quote is as follows:
"On one of the runs a train of 20 coaches, grossing 642 tons, was taken over the test route with the engines at maximum power for as much as was possible within the prescribed speed limits. The sustained drawbar hp was around 2,200 and the 15 miles from Ormside to Ais Gill, largely at 1 in 100 up, were covered at an average speed of 56mph and a maximum speed of 50mph (sic). Fuel consumption for the run worked out at 1.27 gal/mile"
So there you have a 3,300hp prototype diesel pulling a heavy test train over a difficult route. It is probably on the high side, but it gives you somewhere to start. It is also worth pointing out that compared to other diesel engines of the time, the Napier Deltic was much higher revving at 1,650 rpm compared to the more modest 850rpm of the others.