they will then work out your income and expenditure, probably best if you get your original from BR petition, then from your bank accounts for the last few months get an actual i spent Xy on fuel.
list everything that you spend they may choose not to allow certain expenditure.. for example TV media package,
Yes you can appeal but the onus is on you to show why you cannot pay the IPA as it was arranged from the figures you provided on your petition.
change of job, loss of overtime etc is all reasons that the OR will be open to discussion.