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.
For fuel tank capacities, the following data comes from "British Rail Main Line Diesels" Revised and enlarged edition. Compiled by S.W.Stevens-Stratten, drawings by R.S.Carter published by Ian Allen.
The fuel tank capacities can be broadly categorised as follows
Type 1 ~1,000 bhp 400 gallons
Type 2 ~ up to 1,500 bhp 625 gallons
The more powerful classes ate up around the 850 gallon make with the Class 40s only having 700 gallon tanks and the Class 50 being 1000.
I hope this helps. My information stops in the late 70s when my interest in railways was replaced by other things and since I have renewed my interest, I have gone back in time to steam, so I have not updated my diesel library.
Whilst I only skimmed through the Deltic book tonight, I remember as being a great read. One of the articles is by Gerry Fiennes who I remember particularly for his book - "I tried to run a railway". This is the man who brought The Blue Pullman to the Western Region and subsequently bought the Deltics for the Eastern. I heartily recommend both to all "Diesel heads" out there.
If you want more detailed information, back issues of Modern Railways may help or perhaps one of the many "profile" style books that every publisher seems to have on their lists these days.
David