The back seat its-self is not wet but under it is like a paddling pool and now the carpet is wet too. And yes the boot, doors and windows are all shut!
l do not think the sunroof is causing this particular damage as the seats themselves are dry (although l know that the seal does needs changing). How can l check if it is the grommets or rear water pipe etc ?
It is a 1999 (s reg) pug, no water bottle in boot, have small amount of rust on right side of boot but I am beginning to think the plug seals (grommets?) may be the cause as l looked in boot and under seat and they are loose and soaked inside. Thanks for help guys!

Sunroof is a prime candidate if it is got one, or check carefully around the top of the hatchback rubber seal.

could be the rear window washer pipe come off..sometimes cars have large rubber bungs in the floor that come adrift.or lastly the rear hatch seal could be damaged letting rain water in.

Is there a washer tank in the boot? If overfilled they slosh out easily

Try looking at the rubber seal that goes round the tailgate and check it. They do perish. You do not say what year the car is but they can be prone to rust in certain areas. Check underneath as there maybe a hole on the floorpan letting the water in.