So this is what happened!
I recently thoroughly cleaned the bike and emptied under the pillion seat, when I later put a spare lock under the seat it slipped down and stopped the mechanism from unlocking.
Here's the fix to get to under the seat on a GSX-S750 when the key lock doesn't work.
Take the side panels off so you can get to the rider's seat, remove the fixings and lift off the seat, you can now access under the pillion seat. I did notice there is a difference between the GSX-S750 and 1000, the 1000 doesn't have a key in the same position to remove the rear seat. Hopefully, this might help someone one day.
Best idea is to be careful what you put under the seat!