Flossie
The first stage to challenging a Decision is for you to request a Mandatory Reconsideration, this needs to be done in writing to the DWP, within one month of the Decision, to the office that dealt with your claim, have a look at our PIP MR & Appeal guide for details of the process, the PIP area also has template letters that you can use to make the request with.
www.benefitsandwork.co.uk/help-for-claimants/pip You should contact the DWP for a copy of the assessment report if you have not already done so, I would phone them but again follow up the request in writing. Once you have the assessment report you will have a better understanding of how the DWP Decision Maker has come to their conclusions and will then be able to argue against them.
If you at unable to gather all of your evidence by the time that you request the MR then tell the DWP this in your letter and they should allow you a further 28 days to submit information, this includes postage time in practical terms try and get it to them in 21 days.
Gordon