Hope thats the problem, have you checked your exhaust emissions for unburned fuel, drastic loss of economy is usually a leak or injectors needing an overhaul. Also getting fed up with childish pratts hijacking threads with no thought of helping with a problem, am a member of POCUK and you just don't get this crap, thought a forum was to help each other and rip the hell out of each other for any mods made. Not going to lower myself but guess where your V8 should go.
Let us know how you get on after you sort the leak
Davie
If you hate it so much, close the door behind you.