as others above had said you should be getting more than 30mpg out the car... yes progress may not be hugely brisk but you will get where you want to go and go up to the speed limit - just takes a wee while longer to get there is all
Technically speaking the L-series can get up to 60mpg (have done it at 50mph everywhere dying of boredom), most people get 40-45mpg on a run at NSL though - even at 130kph on the continent will still give you that range...
if it is the non turbo charged car - if you have to use motorways at all - you are really really going to struggle, as i think the SDI verses the TDI has something like 15+ seconds to 60....
If it were me i'd look to see why the car is not giving you the economy the consensus suggests it ought to, and then rethink.... maybe pick up something older for less initial outlay? (maybe sorn the ZS in case circumstances change again?)
i know my old nasp 205 diesel with the awful 1.8 lump in it was bullet proof and always gave me over 50mpg however hard i tried to kill it - and boy did i try (driving it 20miles with no coolant as the radiator sprung a leak at NSL
)!