I can't remember whether your's is manual or auto, Phil. An auto should do just the right side of 20mpg unless you do a lot of town driving which might pull it a tad under 20. A long motorway run could see up to about 25. The 4 speed manual adds around 3mpg to these figures.
To improve on these figures the key issue is to raise the overall gearing. For either the auto or manual this is easiest done by fitting an SD1 type LT77 5 speed manual box. Vaultsman has recently completed an excellent thread doing just this. If you want to stay with an auto the easiest swap is to fit a ZF HP22 as fited to range rovers etc. Hold off on that for a couple of months as Ian Wilson is about to try out a gearbox specialist in Doncaster.
Once the gearing is sorted, next up is to sort out the induction system and exhaust, both of which are extremey restrictive. Sorting the induction system is very easy, see:
http://www.classicroverforum.com/viewtopic.php?f=5&t=6727
The exhaust is more expensive, requiring a decent manifold and then a new system to match.
Unfortunately the heads are not far behind. So you'd probably want to add either SD1 or 4.0 / 4.6 stage 3 heads and an RP4 or stump-puller cam. Driven gently that will improve economy markedly - but can you resist the extra horses we've liberated?
Ron and I are both in agreement that there is no particular problem with the SU's until you start winding the revs up a long way. If you do that you first need a much better inlet manifold.
There are quite a few injected cars around using Range Rover systems. Injection per se won't help either economy or power unless the breathing and heads issues are addressed at the same time - which they often are. For me it would be the last item on any shopping list, not the first.
Rather than spending serious money this way, first step is to make sure you are getting the best out of what you have. The V8 is very sensitive to plug leads, so a set of Magnecors are an early investment. Fit along with electronic ignition and a decemt coil. There are numerous notes on how to set up the carbs and check that the distributor is operating correectly.
Hope that gives a bit of food for thought
Chris