Could be the effects of seasonally adjusted fuel. Suppliers typically change the fuel spec during winter to handle the cold weather. Winter fuel does not have the same heating value, hence you use more. Not sure what the practice is in the UK, but it is routine in North America, as standard diesel starts to gel at -10C.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue