HM Revenue & Customs accept that the basis upon which the worker is paid is not a conclusive test of employment status.
However, if you are paid by the hour, it is preferable to avoid using timesheets provided by the end client, as this might suggest that your hours are subject to the end client's control. See
IR35 Compliance Guide - Control.
A genuine business should issue its own invoices.
For IR35 purposes you should avoid claiming expenses in the same way that an employee would. Do not use the client's expense claim forms. Ideally any expenses incurred should be covered by the rate for the job, but if you have to charge separately for expenses, include these on your own invoices.
Avoid any references to "overtime", as this is characteristic of employment.