Vauxhall Astra ridiculous fuel cunsumption

It could be your lambda sensor reading excess oxygen which could be from a whole in the exhaust sucking in air between the engine and the sensor. You could also try replacing the sensor but they aren't cheap, get a scope reading from it first if its possible.

Thanks for your suggestion, I think there is a hole in the exhaust because when I exellerate hard in first gear there is a loud vibration, seems ok when in other gears, and also 10 months ago I got an advisory notice after mot which read part of the exhaust system slightly deteriorated, do you have any idea what should be done, Thanks