2008 Vauxhall Zafira Increase in fuel consumption

I have a diesel Zafira. Up until the last 2 weeks has been very ecconomical. Now the fuel consumption has increased dramatically. Being a creature of habit I do the same driving week in week out.

Consumption in a normal month is around 44 mpg
Now dropped to 36.
Instant read out when normal suggests 50-55 mpg at 50 mph in 6th gear.
When not normal this drops to 25-28 mpg!

On tick over when normal 0.1 gall per hour
When not normal increases to 1.1 gall per hour.

Its almost like driving with the brakes on!
Engine does not misfire or give any other symptoms.

Turning off and restarting corrects it for a whlie.
Does the computer deliver the fuel to the engine or does it mearly indicate what the engine is using?

Please help.