Disco 2 Td5 MPG dropped by a third

I've had my X reg Td5 manual for nearly 3 years now and it's been very good on fuel consumption, last year on a 1,000 mile holiday trip returning just under 35mpg and always averaged 30mpg with normal driving.
Just come back from this years holiday trip, again approx 1,000 miles, and mpg was 23. Was to the same destination both times and driving style/car load haven't changed.

Having changed some bits and pieces to resolve the loss of power going uphills I'd maybe expect a slight change in MPG but not to that extent.
Bit's that have been changed are as follows

• Centre Pipe Delete
• New MAF Sensor (Land Rover Original)
• New Turbo Waste Gate Control Valve
• Cleaned MAP Sensor
• New Fuel Pump (Land Rover Original)
• New Injector Harness Loom/Rocker Cover Gasket
• Cleaned oil from Red ECU Plug
• Freed up Turbo Waste Gate

My lack of knowledge isn't helping at all now but (Dumb question alert) would it need 'retuning' at all, especially with the change to the exhaust?
What would my next investigative steps be on this?

Brakes binding?

Doesn't sound or feel like it, and you can't smell them either, but I'll double check to make sure.

what about basics like airfilter?

Forgot to add that to the list, that got changed at the same time as the MAF sensor.

Could it be that the enngine is running cooler than it should be so maybe a faulty thermostat, seized viscous fan ( running all the time ) or knackered coolant temperature sensor? Have you got access to diagnostics to check some of the above readings?

I don't, but I have a colleague who does. I'll give him a shout and see what we can ascertain.