Complain to the sorting office first. The strip could have been removed by the postman and stuck on the form and any signature could have been put on the form. As far as they are concerned it was delivered and signed for. They won't know different unless you tell them.
Sadly RM don't appear to have the honest staff they used to - and this is happening more and more. Luckily someone honest found it and delivered it to you.

Jan