One place to look immediately is
http://www.zenoswarbirdvideos.com/Images/spit/SPIT9MANUAL.pdf which gives some baselines for fuel consumption for Merlin 60 series powered Spitfires (page 36)
(iii) Fuel consumptions:
The approximate fuel consumptions (gals./hr.) are as
follows:
Weak mixture (as obtained at +7 lb./sq.in. boost and below):
Boost
lb./sq.in. R.p.m.
2,650 2,400 2,200 2,000 1,800
+7 80 __ __ __ __
+4 71 66 61 54 —
+2 66 61 57 50 43
0 60 55 51 45 39
- 2 53 49 45 40 35
-4 45 42 38 34 30
Rich mixture (as obtained above -f 7 lb./sq.in. boost):
Boost
Ib./sq.in. R.p.m. gals./hr.
+ 15 3,000 130
+12 2,850 105
NOTE.—The above approximate consumptions apply for all
Marks of engine. Accurate figures giving the variation
in consumption with height and as between low and
high gear are not available.
I have the Spitfire VII and VIII pilot's notes and the figures are identical
http://www.zenoswarbirdvideos.com/Images/spit/Spit2Manual.pdfgives some basic consumption figures for early Merlin engined Spitfires while
http://www.scribd.com/doc/4599427/Pilots-Notes-Spitfire-V-Seafire-IIIIII will provide some for Merlin 45-55 series engines. Unfortunately there doesn't seem to be much data on given speeds, altitudes and loads immediately available in Pilot's Notes etc; this will take more digging, probably at the likes of the National Archives, Kew for RAE or A&AEE data, or even contacting Roll-Royce archives.