PIP special rules - can we ask for a backdate to the date on the SR1 form?

Hi,
Can you request a PIP backdate to the date on the Sr1 form under special rules (instead of the date claimed, due to a late claim/wrong info given) - Maggies Centre gave incorrect advice, to await oncology appointment where SR1 would be completed, SR1 would be sent electronically to DWP, DWP would then write to relevant person inviting them to apply for PIP. Therefore no call was made to DWP/PIP until approx a week before the oncology appointment when this was mentioned to another organisation who advised this was not correct and to call and claim PIP under special rules immediately. The claim has now only been put into payment from the date of the claim (and not when called Maggies for advice as had already been told less than 12months to live but couldn;'get the SR1 until the oncology appointment which was delayed due to emergency surgery and wait times). Thanks