Hi guys,

Found a pool of water in my boot where the spare wheel was and the driver's side boot carpet was soaking and smelling.

Found where the water is coming from but no idea how it's getting in. It's one of the clip holes near the rear side window. Any suggestions?  Thanks Adam

Think it’d be worth looking at the locut inserts fixings where the rear lights are secured. These are the plastic inserts that fit into the body and the lights screws fix into. 
 

They seal using little foam washers which in my experience simply disappear with age. 
 

Maybe try removing the light on that side and put sealant either around the inserts, or also a bead higher up to divert water away then spray water in the area and see if it’s helped. 
 

When I’ve had this leak the water tended to come out through the boot side trims, I can’t 100% say it’d be the same path as you’ve found so there may be another cause sadly

The water in the boot problem has been going on since these cars were first new. The rear lights is often the cause but not always.

Use the forum search and look for..            "water in boot"