I'm hitting the same problem as described elsewhere on this forum. [I can't link to this]

The glass on the lower RH side of the rear windscreen is coming loose. I can take the boot panel off but what should I use to secure the glass? Can I just use some sealant?

Clear Silicone sealant should do it (have you ever tried pulling am old Fish Tank apart!).
Another option would be Superglue.