I have just been handed a Benjamin Trail NP .177 that belongs to my father inlaw. In has been under his bed for a while I guess.

The picatinny rail has detatched from the rifle. Looks like it was attached with some kind of epoxy glue from the factory.

How best can this rail be reattached?

You might ask that question here https://www.airgunnation.com/
They could tell you what you need to reattach it