You mentioned witnesses that claimed otherwise...does this mean there was no fall? or there was no apparent injury? Hopefully the insured has a record of the witnesses names & phone numbers, maybe even a short written incident report taken by the insured. Several stores/businesses practice that, and it helps.
After contacting the claimant & scheduling an appt for the recorded statement, I might try to get recorded statements from the witnesses. Have your scene photos with you, and have them well labeled and permanently numbered, and refer to them by number on the recorded statement so the witness can give you the most accurate answer "to the best of their recollection".
If all of the witness statements jive, this will give you a better line of questioning when taking the claimant's statement. Remember this might just be that there was no "apparent" injury in the minds of the witnesses...no blood, or obvious visual injury...but something that came on a few hours later such as a muscle strain, etc.
Find out if the carrier you are working can run an Index report. Many carriers participate and report each and every claimant/insured that has filed an injury claim, including work comp. If an individual has prior injury claims, a report will come back telling when, where, what type, who the carrier was, etc.
Last thoughts, when turning the recorder off...again state the date and time. If you are ever interruped (the claimant gets a phone call), announce the date & time and that you are taking an interim break. State the date & time again when you re-start. If your tape is ever subpoenaed, this will CYA if accused of editing. If there are any other people present when taking the statement, introduce them early on & have them acknowledge their presence. Ask them to remain silent, but if they do interject they need to state their name first & the reason they are interjecting. This is more CYA, and also makes it easier for the transcriptionist if the carrier decides to have the tape transcribed.