Anyway, After removing the new underfelt I found 2 cracks that are just about to join up in the areas shown below. The black tar paint makes a decent photo of the car impossible at the moment.
[frame]
It looks like the new floors that were put in a few years ago by the previous owner had some unresolved issues with flexing and now useage has made the cracks expand. I guess every time the seat was sat in it would make the panel 'pop'. The gap under the longest bit of crack is big enough to get a finger through.
Interesting thing is that it passed an MOT last week!