Expected MPG? Range Rover sport 2.7d.

What would an acceptable MPG be for general mixed driving? I am getting just over 20MPG (l322 2.7d 108000miles) and wondering if I can do any basic servicing things to increase this a bit.

I think my battery might be underpowered, gonna sort this on Monday, would this contribute to poor due economy?

thanks in advance

If the battery is more than 5 yrs old it'll be on its way out and shouldn't effect mpg. Most garages can test a battery and the cold winter will certainly find a duffer.
Air filters are a biggy on diesel engines. The oil and filter is better sooner than later due to the crap produced from the combustion process.
The gearbox oils, transfer box oils and diff oils help to maintain the health and longevity of those components.