Thanks for the advice.
After spending most of the morning on the phone to them it's looking like I will leave a day earlier and go GND-POS with carib air, then have a 9hour lay over until I go POS-UVF-LGW.

Other alternative they proposed was to go to UVF as scheduled then spend a night there before carrying on the next day, but I can't do that due to work issues.

They don't yet know the reason for my issues, they said it was rerouted on 13th April , coincidence that it was at exactly the same time I paid to pick my seat on outbound? Yet the 2158 has always been scheduled at 1750, the same time as my elusive 2156 flight. Anyway, they are looking into the reason.