fuel costs ... mpg

does 200 miles for around £50 of diesel sound normal in a 1998 2.5 tds frontera with the vm engine?

That's what 36,37 litres depending on where you are?
Something like 32 mpg then. Sounds about right although it will depend how you drive. A-series are heavy and in-aerodynamic so you're never going to get stellar mileage.

Long motorway cruise at 60mph should a bit more miles per gallon, decreasing the faster you go.
Short stop-start stuff around town and you'll get less as its less efficient.
Plus if you have a lead foot, that doesn't help.

is their anything mechanically i can do to improve the mpg, already changed fuel and air filter

tyre pressures.
remove any unused roof racks etc

and i have done a 300 mile trip in my swb 2.0 petrol for about £45
so you should be able to do 200 for £50 no probs

just have a light foot

Clear as much junk from your boot as possible,
Lose the roof rack to lessen drag,
If you're really keen, lose the spare wheel and bull bars to save even more weight,
Regular oilchange to keep the motor sweet,
Properly inflated road tyres, rather than all-terrain or mud-terrain types.

I will second that with the tyres if you can afford to get your another set for off roading.

try lowering it slightly to increase mpg mine dropped when i lifted it . as for tyre pressures my roads are at 34 psi