Eh up,
Guydickinson- I’ve read your thread with interest , I’m as intrigued with this as Herts.
Can I just check that both your bills are all clear after your first rinse out?
The

that collects in these gullies defies belief and if you’ve cleared stuff out, it might not be everything as you will have left some in unless you’ve had the scuttle off. Just check when it’s dry water runs cleanly through the bills again and they’ve not blocked again.
If it does then there’s very few places it can stay under the scuttle, you can take it off and see for yourself, its an easyish job. I can’t see how water is sitting in there and staying enough to hear the sloshing
I’m stumped other than suggesting above, or checking cill drain points are clear ,on the doors too and or taking plugs out of monocoque ( ones near jack points or they are on mine) to see if the water is trapped anywhere else.
Other is water in boot well, but if the sounds definitely in the front I’m at a loss
Keep us posted and good luck