Refer to CAP 697 Section 2 page 6 ...

Question conditions, 8000ft, OAT + 13º C

8000ft, ISA is -1ºC, so temperature condition is 'ISA +14ºC'

We need to interpolate between ISA and ISA +20ºC

From the table, 8000ft:

ISA ...... Fuel flow 71.1PPH, Man Press 22.4

ISA+20º. Fuel flow 68.5PPH, Man Press 22.4

Interpolating ... (71.1-68.5)x(14/20) = 1.82

So fuel flow = 71.1 - 1.82 = 69.28 PPH

Also, refer to CAP 697 Section 2 page 1 .... fuel density 6 lb per US gallon (unless otherwise specified)