I use to get more than that in a work Ranault traffic (Vivaro) but, I think those on the scheme are electric now with 145 mile range, depending on how far you travel.
Is there any health reasons you can think off that makes the Tourneo unsuitable? If not you wll have to tell Motability you can simply not afford to run it, therefore you are not getting the intended use out of the vehicle and know the facts your next choice of WAV will be cheaper to run (it may be worth making some notes f the different vehicles economy)