I am considering purchasing a diesel car due to better MPG. One thing
that puts me off is after only ever driving petrol cars since passing
my test I am concerned that it could all be too easily to accidentally
fill it with petrol by mistake. Any mistake like this would wipe
anyway savings on fuel costs, only has to happen once. Do you have any
tips or tricks you use to minimize this risk?

Surely they could of invented some kind of device/mechanism that would
only allow a diesel pump nozzle to fit into a diesel tank?