uPVC door bolt?

Note: Originally posted in "Builders" Forum - no takers.

Does anyone have any unusual ideas for some sort of old fashioned bolt on an unglazed composite uPVC door?
My elderly neighbour is getting a new back door but wants the peace of mind of bolting the door from the inside overnight.
The double-glazing salesmen try to assure her that the new Euro cylinders are so good that she doesn't need .... ya de ya de ya.
She wants a bolt or something else that she can see and touch.
She'll have a chain but that's not enough for her .... she knows that a small bolt cutter can be poked through the partly open door.
We all know that anything screwed to the plastic will:
a/ fall off
b/ invalidate the warranty.