Bill Woods wrote:
I have a PIR for sensing people in our garden. I want to have the
PIR located behind glass or plastic.

If the PIR is behind glass then the glass blocks the heat which
triggers it. I image a clear plastic sheet would also block heat.

Is there some special plastic (like that used in the PIR's lens)
which can transmit heat?

Thin polythene.
It's what the lens is made of.